My bub was born at 39w and was just 3kg (3010g) or 6lb 10oz and we were in 0000 for a while. It all depends... This one will be born anywhere from now on so I have some 00000, 0000 and some 000 just in case..
As for the will, my dh and I got ours done in 2006 (pre kids) as he was going OS for work. Our "future kids" were written into it, so no changes needed here. It is a good idea though, especially for care of kids and finances incase something (god forbid) happens to both of you.