It depends on what level of training you are at and what you prefer.... personally, I hate those TT bits.... here's a link
http://www.todayshorse.com/Articles/...thTomThumb.htm
What I do, is make sure the horse knows how to flex with the halter, then put in a mild frenchlink type snaffle... the french link I will use for building the foundation, and I don't move up until the horse has a good idea of everything I ask.... then I move up to a billy allen w/ 5 inch shanks, and then a mullen mouth w/ loose 5 inch shanks, and then finally a low port curb w/ 6 inch shanks
Most horses will fit in a 5" bit, but if you want to measure, put a piece of string in your horse's mouth like a bit and measure the width of the mouth plus a tad bit over.....