HP of tractor and size of mower - there are more issues than just that.
Will the tractor hitch handle the tongue weight of the mower. Some utility tractors may have the hp to turn the blades, but not much strength in the hitch attachment.
Mowing dead weeds and grass or 2" diameter trees and 8 foot tall briars?
Mostly level ground or steep hills. Weight and stability of the tractor, center of gravity, number of driven wheels are important even if the hp is sufficient. Sliding down the hill with a mower is not good. Turning over can be deadly.
Speed of travel. If the mower is too wide, you might be able to pull it going very slow. Are you a patient person?
Important to have the mower adjusted correctly. Mower should be set so that the blade tip is lower to the ground at the front of the mower and higher at the back. Then, the mower does all the cutting at the front forward edge and then rises above and rides over the cut grass farther back. If the back is lower than the front, the blades are cutting each stem of grass multiple times since the blade height lowers in the back. Are the blades sharp or worn out like mine? Obvious effect on hp and fuel and wear on the blades.
Will a 75 hp tractor handle a 15' mower? Yes, but not very well and maybe not very safe.