Resizing a shirt..

[fcku-thesound.livejournal.com]

ok, tuesday night I saw Dave Matthews Band & bought a shirt. The shirt is a medium and I usually wear an x-small or a small, I was planning on cutting up the seams and re-hemming it and what not, but the shirt has no seams going down the sides, and it's hemed @ the bottom. So I was wondering, what is the best way to resize it? Should I make seams going up the side?

help!!!