What factors determine the size of a friction clutch?

Clutch size depends on torque capacity T = mu x p x n x 2pi x (ro^3 - ri^3) / (3 x (ro^2 - ri^2)) for uniform pressure assumption, where mu is friction coefficient, p is allowable pressure, n is number of friction surfaces, ro and ri are outer and inner radii. Design considerations include heat dissipation capability (temperature rise per engagement), service factor for shock loads (1.0-2.5), wear allowance, cooling arrangement, and actuating force required.