Mine's only two months, but he's about the size of a five month old.. LOL I can measure him anyway when we get home if you'd like. <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/smile.gif" style="border:0px solid;" title="smile">
Okay I've turned the house upside-down and still can't find the measuring tape. Grrr. I can tell you the measurements I used for his current ones, which I'd say fit him best from about 11-16 lbs. (He's 15lbs 6oz now and they still work but not for much longer.)<br>
We tend to use them with prefolds, so I don't know how much that would change your numbers.