Because they have what I call a "boomp boomp" tail (really big) and oversized heads, it must be big enough to get over them or up from their legs, and it must be either an A-line shape, or have alot of elastic. You can't buy too big because then the sleeves or the pants are too long. Alot of times shorts look like pants on them.
For Disneystore.com I purchase the 6 month sizes.
For Target, I got 12 months in everything except the Monsters t-shirt which only came in 18 months (I might have to hem it). They were sold out of everything smaller.
I have not gotten any Walmart clothes for them yet but I'm going there tonight to see what they have.
Baby clothes aren't really proportioned right for the bears so I found that eyeballing it and just experimenting is best.
For Duffy and Shellie May, I ALWAYS get the newborn clothes but because they're only 17" and newborns are usually longer, I have to be careful to either hem, or again, get stuff that has buttons for the neck hole to be big enough, and hem it, or alter it, which is why I stopped doing that.