Its so hard to tell. I've found sizes vary a lot between stores like Target and Walmart, Petsmart etc. XS in one store is too big for my littlest and sometimes a small somewhere else is to small!
Most of the time though, small works for little ones around #5 and under. Depends on how long they are and how "plump"
There are a lot of great YT vendors here that make clothes for our furbabies that make them to the size of your baby. Check out the YT products for sale section
