Gerber definitely runs small, I also think that Gap and Old Navy do also. My LO is also tall and skinny... I find that the actual Carter brand onesies seem to be the longest.
It seems like Gerber runs a little smaller than Carters, and they both run super smaller than Circo... DD can fit pretty well in NB size Gerber/Carter but she is swimming in Circo NB.
Re: What clothing brands run small?