Eric is correct. You will need an 8mm hex bit. However, you can get these cap screws out without bolting to the wheel.

Thread two lug nuts into the hub and get a two foot bar and shove the bar between the two lug nuts. The hub rests on the two lug nuts.

Then take your breaker bar, with 8mm hex bit attached, and pull on the cap screw, while simultaneously, with your left leg, prevent the rotor from turning, by stopping the bar from rotating. It's a bit awkward, but, I've done it three times now and was always successful.