for you, i'd recommend going with 8" front, 6" rear. You could probably get by on a 6" front, but the front is where you get most of your actual stopping power so that's where i'd recommend having the larger rotor.
i used to ride diablo regularly until I relocated to SoCal, so the 8/6 combo should work out great for you based on your weight and diablo's terrain. If you're uncertain, then go with 8/8 to be on the safe side.. Doesn't avid also make like 7" rotor?
tmeyer you also have to remember your running codes man. if i was running codes i could get away with 7" rotors or even 6" all around. with juicy 7's i wouldnt feel safe with anything less than a 7" rotor, and im only 150 lbs-155lbs with gear on.
The majority of people will get enough stopping power with any sized rotor. The bigger ones, however, not only deliver higher leverage, and thus power, but also have higher heating and cooling capacity, preventing premature brake fade during long braking descents. This varies depending on many factors - brake type, pad type, braking habits, brake usage balance (F/R) etc. Basically a lighter rider will get away with a 7/7 combo, but everything depends.