I've got 245's on the front of mine w/ 17 x 9 cobra R's, I wouldn't go over a 255. I'm pretty sure a 275 on my car would stick out and rub the fender. Car is lowered though.
EDIT: if you go with BFG drag radial, EXPECT them to wear out QUICK. I ran a set of 275/40/17's DR's, and depending...