Croquet mallets have changed considerably over the last 20-30 years. In the entire history of croquet up until around the 1990s, you would be hard pressed to find a mallet that wasn't completely made from wood, save for some kind of toughened material on the striking faces. In the late 90s to early 2000s a number of new mallets hit the market incorporating peripheral weighting (a concept borrowed from golf) and a number of fancy new materials such as carbon fibre. Nowadays, peripherally-weighted carbon fibre mallets dominate, and their effect on the game has considerably changed the ability of players world-wide.
When choosing a mallet there are a few factors to consider:
A "standard" mallet that you may find in the shed of a croquet club is normally 1 yard long (36inches / 91cm). The reasons for this is that it is a somewhat happy-medium length and can mostly be used by anyone, and conveniently is a useful marker for lining balls onto the yardline. However a standard mallet length is seldom the correct length for most players - the correct length is very much dictated both by your height and your grip. For example, a tall Solomon-grip player may require a very long mallet of 42" or more in order to swing comfortably; conversely a shorter player that uses Irish grip may be a lot more comfortable with a handle length of 30". In general, Solomon grip players require the longest handle; Irish grip the shortest; Standard grip somewhere in between, but again comfort has a lot to do with this choice.
This is one area where the standard specification has changed over the years. 20-30 years ago, most mallet heads were around 9" in length. Over the years this has increased and it is now quite common to see top players using a 12" head. Most club players will also be using heads of 10"+. The head length you choose can have a big effect on the arc of your swing - the longer the head, the flatter the swing needs to be in order to have a clean strike on the ball. When switching from one head length to another (particularly when going from a shorter to a longer head) there will be a period of adjustment while your swing and technique adjusts to the new length. Generally a longer head will be more accurate for hitting a ball in a straight line, both because aiming with a longer head is easier, and also because a longer head will naturally mean that some mass is distributed further from the axis of rotation (the handle). On the other hand a shorter head will give you better stop shot ratios and generally make it easier to control the back ball in these types of stroke.
This is a less critical decision and more a matter of personal preference. Shot outcomes and croquet stroke ratios are not affected by the head width in any way. Some top players use mallets with very narrow heads which may be little more than 2 inches wide because they are very confident of striking the ball with the centre of the mallet face. On the other hand, a wider head width would be recommended for a new player as it would probably result in a lower number of mis-hits. The one area where a narrower mallet head can provide a true advantage is with hampered shots - for example if you run a hoop but the hoop ends up hampering your next shot - sometimes the attempted roquet would be played with the mallet head between the uprights of a hoop, and a narrower mallet will assist with this.
A croquet mallet 100 years ago would likely have had a circular wooden head. However these went out of fashion a long time ago in favour of a square cross section. In reality there is no difference in the playing characteristics between square and round, but the square section has a couple of minor advantages. Firstly and most obviously, the square mallet can stand up on its own. Secondly, severely hampered shots are slightly easier with a square endface. Finally, if a player mis-hits a ball by catching the ball with the bottom edge of the striking face, the shot is much more likely to still go in a straight line with the square face compared with the round face.
Interestingly, around the turn of the millennium, there was a resurgence in popularity for round-headed mallets - not due to fashion, but rather due to the emergence of carbon fibre in mallet manufacture. Carbon tubing is most easily and cheaply manufactured in a circular tube shape and hence this was the shape that many early carbon fibre mallets took on. In recent years, square carbon has become more readily available, and as a result we are seeing fewer circular mallets.
This is a very important factor. Standardly a croquet mallet is around 3lbs. A heavier mallet will make it easier to hit a ball in a straight line, and also make playing half rolls, full rolls and pass rolls easier; a lighter mallet will have the opposite effect and will give more control and better stop shot ratios. When choosing a mallet weight, it is important to try out a number of different mallets to find what works most comfortably for you. Generally players will look to choose a mallet weight which gives a good compromise between accuracy of touch (lighter) and accuracy of shooting (heavier).
As well as the overall weight of the mallet, another factor in this is the balance point. Traditional wooden mallets would have a balance point of around 1:5, meaning that if you balance the mallet sideways on your finger, it would balance around 1/5th of the way between the head and the end of the handle. In recent years, the fashion has been towards mallets with lighter, carbon fibre handles that have more of the overall weight concentrated in the mallet head, and consequently a balance point of around 1:8 or 1:10 (or even higher) - this enables a pendulum swing to be achieved more easily, as the majority of the weight will be concentrated at the extremities and far from the hands.
This is a factor which is usually overlooked, but nonetheless it should be given some thought. Generally a mallet handle when viewed from above will be oval or a rounded rectangle shape. It is possible to request a mallet manufacturer to offset the rotational alignment of the handle so that the handle is aligned in a position that is most comfortable in the hands.
To find your optimal alignment, pick up a mallet with your eyes closed, and hold the handle in the most comfortable position possible - chances are when you open your eyes, the mallet head will not be pointing directly forwards. A number of top players, such as Reg Bamford advocate an offset rotational alignment.
Not as complicated as it sounds. This is a concept that aims to concentrate the majority of the weight of the mallet in the head, but particularly at the extremities of the head (the striking faces), in theory enabling the mallet to swing more easily in a straight line, and also reducing the impact of an off-centre strike. This concept has been in use in croquet for decades, and any mallet, regardless of the construction material, can benefit from it. For a wooden mallet, this could be achieved by the manufacturer drilling out sections of the underside of the mallet head that are near to the axis of rotation (the handle) and adding the weight back in (via brass / tungsten plugs) closer to the extremities of the head. This concept has been taken to the extreme by the use of carbon fibre mallets, where the entire mallet head is essentially a hollow tube or box (sometimes filled with foam or a dampening material to improve the sound), with 90%+ of the head weight being confined to brass or steel plates just behind the striking face - or in some cases acting as the striking face itself.
Years ago, a traditional mallet would have been constructed entirely of wood. Lignum Vitae, the densest wood is an ideal candidate - it is a wood dense enough to withstand continued striking of a croquet ball without the need for an additional endface to be attached - although for added protection and to stop the wood from splitting, the Lignum would normally have a brass ring wrapped around its circumference near the end. Most wooden mallets are made from less expensive and less dense woods, such as rosewood, and use tufnol or other hardened materials for the actual striking face. Actually, from a peripheral weighting perspective, it is advantageous to use a less-dense wood for the mallet head, as it means that extra brass or steel weighting can be added directly to the extremities of the head in order to bring it up to its chosen overall weight. For a wooden handle, ash is a popular choice for its flexibility and durability.
While wood is still a very popular choice for mallet material, today we see more and more mallets being made of more futuristic materials such as aluminium, graphite and carbon fibre. Using these light and strong materials allows peripheral weighting to be incorporated into the manufacturing process itself.
For a list of reputable mallet manufacturers, see the Links section on mallets.