Changing front Rotors?

What's up,
Today i was trying to change my front rotors because their kinda warpted. So i took off the caliper and i got off the 4 bolts that hold the rotor to the plate there. how the hell do i get that plate off??? I'm guessing it's that damn huge nut but that's a huge nut. If i need to take that nut off what is the correct size? I didn't bring my helms manual so i dont know off hand. Thanks...plz feel free to give tips on how to do this fast and easy. PEACE