As everyone probably knows by now...hahaha
I went to Target this morning, and bought some stuff, well alot of stuff.
The little vest that was $2.50 has sizing info that reads...
"Measure from the base of the dog's neck to the beginning of the tail.
X-small: 6"-9" (2-6lb)
small: 10"-13" (7-12lb)
medium: 14"-16" (13-20lb)"