The sizing guidelines for bikes will give you a general idea of how to best fit a bike, but there are some other considerations you'll want to take into account. The most important one is your full inseam (not pants size) with shoes on. Particularly when mountain biking, but potentially for other styles of riding as well, you want to make sure you have enough clearance (at least a couple of inches) over the top tube if you have to come out of the saddle and stand over the bike.
For the DRT 1.1, the standover height of the large frame is 29", the XL is 30". Adding a couple of inches for extra room, you're looking at 31" or 32", respectively. Generally speaking, I would go with the smaller size on a mountain bike, just to be sure. If you're inseam is really long, say 33"+ then the XL would be the way to go.
Personally, I ride a Co-op Cycles DRT and am 6' tall. I ride a size medium because I have short legs (30" inseam) and a long torso. The medium standover height of my bike is 28.5" and when I'm wearing shoes I have about two inches of clearance over the top tube, which has been just about perfect for the three years I've been riding my bike.
Hopefully this helps!
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