Bought the 4T size for my son who turned 4 this week.  He's a slim child, so the 4T size fit fine.  He has just outgrown his 2T Gerber training pants, which he was using as underwear, so I agree that the thomas briefs run small.  The designs are cute, but the only problem we are having is that fly on the briefs is shallow, and he keeps popping out of it.  I tried double-sided tape on the fly front this morning, we'll see how long that lasted.