So, "harrowing", I read that as a disc harrow, but I might be wrong. 25hp should be a 4 to 5 ft disc, a 35hp should be good with a 6 ft. If Harrowing is more like a chisel plow, or something else, soil will be the main factor.
With ground engaging work, like a plow, disc, cultivator, chisel plow, ect, you need the power to run at the speed it works best at; but what works in my sandy soil, may not work in yours.
If harrowing is more of an arena drag or spike harrow drag for breaking up manure, even a 25hp should pull a 7 ft no problem.