Interestingly, we were at Children's Place a few weeks ago and overheard a similar question. Our son is getting into 5T for the same reason, he has really long limbs and is outgrowing things in the arms/legs before the waist.
The store clerk said that the Boys' 5 is actually smaller than the 5T. We kind of scratched our heads on that one......we didn't ask for clarification.
I don't know if you shop at Carters, but we've found their sizes to be more generous in length and have been getting as many things there as we can with the 20% off coupons they offer.
We've also had good luck at Kohl's, and the Boys' Department there has a much better selection than the Toddlers. But, finding the XS seems to be trickiest.