Where can I find fitted shirts for a thin/tall guy?

Alright, soooo, I'm about 6'4..6'5ish? And pretty thinn. I'm muscular..toned, but not big and buff, and I'm definitely not skin and bones. Probably considered athletic?

I'm white, which, without stereotyping can often justify that I have little-to-no backside. :P

T-shirts, I often wear a small, または a medium.

THE POINT:
Some smalls and most mediums tend to "float" behind my back/butt, but fit my chest and arms well. Some smalls don't float, but they are a bit short, and some smalls fit perfectly. HECTIC!

Now, this barely ever applies to a certain "brand." Some American Eagle shirts fit well, some don't. Some Hurley shirts fit perfect, some don't, well...most don't..

I'm planning on getting a LOT of my stuff at Forever21 as I've become a bit indie/street casual, with my 音楽 and style/attitude, and I'm about to be a Freshman in College. I was wondering how things would go with shirts there, as well as sweaters for the fall.

Sorry for soo MUCH detail, but it's a bit err-say..required, to get my point across. :P

Thanks in adv. :):)