Re: 6 or 7\' brush-hog on an L4400
The biggest single consideration should be weight. It does take a lot more HP to run 7' over 6'. But look at weights of 6 and 7 foot mowers of the same brand/model. The 7'er is generally 40 to 60% heavier. Add in the fact that the weight will extend back at least another foot behind the tractor. Most medium-to-heavy duty 7' mowers will weigh from 1200 to 1800 lbs (and even higher)
Anything short of mowing your lawn, I'd stick with a minimum of 50 HP for a 7' mower. 60 would be better.