I bought Kimbo's in a small pet shop in Houston. The brand is East Side Collection and he wears XS. It's very true to size.
Found the same thing @ BaxterBoo.com with the same price that I paid. The measurements are listed for the various sizes. Here's the link:
East Side Collection Fashion Aquatic Life Vest Preserver - Pink and Green at BaxterBoo
Here are a few pics of Kimbo in it. Lol, there weren't any boy colors there the day we went, so I got him the pink/green. DH and my sons make fun of him losing his man card in pink, but I think he wears it well.