There are a great many factors that go into a racing chain. In my opinion, the best thing to do is to purchase a chain from Art Martin. There are folks who post here that file several chains every week, and they purchase their racing chains from Mr. Martin because they cannot duplicate his work. Filing a chain for racing is an art form in itself, and the instructions cannot be easily put into words, at least by me. In addition, racing chains begin as "square chisel" chains, so one must start by learning how to properly file this type of chain, a skill not easily aquired. Also, many racing chains are filed in ways that could weaken the chain if the work is not properly done.
That having been said, In answer to your question:
Racing chains are always "square filed chisel" chain. The teeth are filed back to narrow the kerf. As you look at a chain from the top, you will notice that the cutters are not parallel with the bar, they are at an angle. As the cutter is filed away, the cut will become narrower, and the saw will cut faster because there is less wood to remove. An expert like Art can further modify the angle of the tooth to get it to cut even faster.The gullets between the cutter and depth gauge are opened up. The side plates and rivets can be filed to lighten the chain. All of the angles on the chain are filed in the way that will provide the fastest cutting, and, of course, the chain is as sharp as a person can get it.
if I were filing a chain for racing myself, what I would look at doing is starting with a chain that has had the teeth filed back past the front rivet. Sharpen the chain as best and as consistently as you can. Do not lower the depth gauges too far, this will slow the saw and is dangerous.
Carlton is a good chain to start with. Make sure that your racing chain doesn't have bumper tie straps, this hampers the clearing of the chips.
I will leave the chain size question to others, I can't get the .325 stuff to cut worth a darn in a race.
I hope this wasn't too basic for you.
To learn more about square filed chains and racing saws, check out
www.madsens1.com