the secret to having longer shirts/hoodies/pretty much any clothing article
option 1: get it normal size or a size down, and when you wash it, stretch it long-wise before putting it in the dryer(or hang dry for even beter results)
option 2(for things that dont stretch and/or have a zipper): get it a size bigger and have your mom/girlfriend/grandma/self tailor it down a bit on the sides.
as for getting things online without trying them on, obviously get a size up if your unsure.