Nikko was 5lbs at the time of his neuter, and we used newborn onesies. The front of the onesie was on his back, we snapped everything shut, and his tail fit perfectly through a little hole we cut through the "front crotch" part of the onesie.
Wal-mart had a really cute pack of five for I think $11 - they all either had puppy dogs on them or were striped. One of the puppy dog ones said "Daddy's Champ" with a pup on the back - my DH loved that one, lol.
Lauren & Nikko