I always find bits to be a bit trial and error. They are measured in inches, with a quarter of an inch difference. To give you an idea of size, my cob is a 5 3/4 bit and my TB a 5 1/2. They need to be loose enough that they don't pinch their mouths, but obviously not so loose that they're uncomfortable. And different bits fit differently. In his Dutch gag, Jacob needed a 5 1/2. The French Link snaffle had to be a quarter of an inch bigger.
Try a couple of sizes (I heard from a friend that some companies do tester bits, made from rubber so you know what size to order... not come across them myself), and if possible, get your YO or RI to check that it's a comfortable fit. When I fitted Salsa's new French-Link snaffle (he came without tack), I had to try three different sizes. The difference wasn't a lot, and I had to ask my RI to check it fitted correctly.