This particular link has a picture of the different sizes so you can get an idea:
Latex bands for dog bows I haven't bought bands from this site so I can't comment on them, but you will see the different sizes. I read that size isn't too important, with a smaller band such as 3/16 you might only have to twist once, with a larger band such as 1/4 you might have to twist twice. I would not recommend anything over 1/4 inch for thin hair though. The 1/4 inch size is also the size you see on most bows.