My son was 9 lb 11 oz when he was born (you know you've got a monster when the doctors and nurses cry "My God, he's huge!" when your baby pops out), so I knew from the start that infant clothing sizes would be useless to me. Nugget never wore the newborn sizes, and at two months he's outgrown most of the 0-3 month sizes. My reaction to most problems is to obsessively organize, whether that's a reasonable solution or not (I kept a spreadsheet of every single thing I ate or drank while pregnant). I've therefore compiled a lot of information about the sizing of baby clothing, and you get to benefit. I'll update this as I collect information about additional brands.
I’ve been promised information for the following brands: Junk Food, Little King, and Gerber. I’ve contacted representatives for the following brands and have yet to hear anything: Baby Essentials, Calvin Klein, Izod, Juicy Couture, and U.S. Polo Association. I have not yet been able to find contact information for the following brands: Pine Sports, Lullaby Club, Kiddy Cats, and Baby Gear. Representatives for the following brands were unable to provide information: Dr. Seuss Enterprises. I’m working on the Babies ‘r’ Us brands--Amy Coe, Koala Kids, and Miniwear. A Toys ‘r’ Us representative told me to contact my local BRU store for the information, but the closest BRU is a trek and I doubt a phone call will be productive. Stay tuned.
[3/16/10 Edit: I'm afraid I never got the rest of the information promised above and my son is now an 18-mo-old linebacker whose outgrown all these sizes, so what you see is what you get. A couple general points I've found from experience: Gap and Old Navy pajamas are designed for really long and skinny babies, no good for my tubby tubs, and Carter's sizing tends to vary. Seemed to me the "Just One Year" stuff in particular was sized differently than other Carter's lines. I do love Carter's though. Carter's and Target: god's--or at any rate, capitalism's--gift to moms. Anyway, welcome to my blog. Statcounter tells me the vast majority of my vistors are here for this chart. Have a look around while you're here!]