Like others have mentioned, order a size up (especially if your kid grows like a weed like mine does!).  I ordered 18 month for my daughter when she was just 9-12 months size in everything else.  I've gotten alot of use out of them, and just now have had to go up a size, as she's settled into 2T clothing. (I can still get the old ones on her though, the sleeves and legs are a little shorter, and it's a little harder to get on her now, as the snaps do not go all the way down to the leg cuff, so there's some bending of legs involved these days).  I figured that from 18 month to 24 month, it would be bigger, however I was a little dissapointed that it was only bigger in length, not width.  If your baby is quite a bit chubby, these are not for you.  Luckily, mine is not chubby, except in the thighs, and the 24 months seems a little snugger than I was hoping it'd be. :(  Also, it seems like it's thinner fabric than the 18 month size, which is really wierd.  I've only washed them once, though, so maybe it'll fluff and soften over time.  I don't remember if that was the case for the 18m ones, but I do think the fabric got softer over time.
Overall, they're great constructed coveralls.  The snaps close well, it washes over and over again without issue, and the baby has put some real use out of them, and they're still going strong.  I love them, and have purchased twice now.  Wish they made larger sizes.  Just keep in mind, if you have a really chubby-tummy baby (all babies are different), these may not be for you.