I kept wondering when he was going to fit in it (it looked huge), until I looked up Gymboree's sizing chart and realized the 3 meant 3 years old. In hopes of not making that mistake again, or the chance of missing and adorable outfit I looked up the sizing chart for the most common baby clothes.
***UPDATE 2/20/13*** This post has been gaining a lot of pinterest traffic. I just wanted to let you know that all sizing guides came from the companies websites. Gap and Old Navy are grouped together because their website's have the exact same listings. Hope this chart helps you stay organized!
Here's a more practical guide (not so cutesy).
Does anyone else have a hard time moving their babies up to the next size clothing? I do! It is just so hard to admit he's growing up that fast! Baby Boy has high water pants and stretched bodysuits before I finally switch him over. It is fun to see him in new outfits though!