It’s not the size of the crush; it’s how you use it.
The crush of the grist results in different size particles that give the brewer the range between the speed of sugar extraction and the ability to extract overall. A small or fine crush will allow sugars to be converted faster by enzymes but can create a stuck mash or unwanted husk tannin extraction. A coarse crush will allow for a better flow of wort and lauter but will not be converted as quickly. While the degree of the crush can have a significant impact on the speed of conversion, the difference in the total amount of final starch extract is fairly minimal. In the end it is up to the brewer to determine what is best for their system, or whatever the local homebrew mills for you.
Again, if the end yield is roughly the same, why use different size crushes? One factor is time. For home brewers, time is usually not a luxury, as no one wants to spend over two hours on mashing and lautering like in commercial breweries. Creating a finer crush allows the ability to extract the desired starch in a reasonable mash/lauter time frame. The drawback is the degree of fineness your mash tun setup can handle before sticking your mash and/or compacting your grain bed. Some factors that can increase your system’s ability to handle a finer crush are the size of the holes in the false bottom, the speed of lautering, use of rice hulls in the mash, types of adjuncts or non husked grains, or if you are using a BIAB (Brew in a Bag) system, among others.
Another factor to consider is the insoluble grain husks. The integrity of these husks is better-maintained the coarser the crush, so finding a middle ground is important. The husks allow your wort and sparge water to flow through the grain bed in order to extract the sugar, as well as preventing the bed from compacting. The holes or slots of the false bottom/grain bin, or density of your grain bag can clog your mash if the crush if too small. The speed of the flow will also determine the effectiveness of the lauter. Go too fast with a fine crush and you may stick your mash. With a coarser crush, higher flow may channel in a non-uniform manner through the grain bed, causing preferential flow and reduced efficiency.
Rice hulls are commonly added to the mash to help with lautering, especially when using viscous (rye) or non-husked grains (wheat) or flaked adjuncts (flaked wheat/oats/barley/maize/rice). When making a beer with wheat, flaked adjuncts or rye, the typical rule of thumb is to add at least 5% of the adjunct grain’s weight of rice hulls to the mash. The newer practice of BIAB has allowed brewers to reduce concern of too fine of a crush while increasing efficiency because of the ability of retaining all the grain in a bag that can be removed from the mash prior to the boil. It can be onerous to lift the bag out and collect all the drippings, but it has become a quite popular method.
Overall a good crush involves exposing the endosperm and leaving the husks intact, not just cracking the grain. Roller mills are the industry standard and they typically consist of two or three rollers that are fixed or adjustable in spacing. The size of the crush is something that may be the same for every brew, or change depending on the type and size of grain, or process of brewing. Many brewers just rely on their local homebrew store to accommodate the right size crush, which can be very helpful if you do not have a mill at home or are getting inconsistent results. In the end, the size of the crush is not as important as how you intend to develop the wort and the style of mashing/lautering. The main goal is a final product you can be proud of and enjoy with friends, family, or just yourself. Cheers!