Some people say they put the onesie on backwards with the snaps in the back and just stick the tail out between the snaps. I think this makes it harder for them to get to their wound, but I didn't read about them until after Joey was neutered, so I never got to try it. I just love the pictures though, and double pictures would be even better.
Depending on the size of your dogs, I think newborn and preemie are the most popular sizes. Remember, no baths for a couple of weeks!