how do u know what size to get? I'm 6ft, 184lbs. I know they need to be tight so would i go a size smaller, say a medium? I have 2 rash guards that don't have the size on them so that's why i'm unsure. also, what's a decent rash guard to get? may just go to modells and grab a few under armors, but are there any differences between ones for grappling/running, etc.. i should really know all this stuff by now, but i mainly train with a gi, or basic t shirt/shorts. any input is appreciated, thanks.