Mowing 4-5 acres of mow-it-three-times-per-year pasture with a 6' bush hog at a typical speed of 3-3 1/2 MPH takes about 2 to 2 1/2 hours. I chose that speed as that's what speed I run at when I knock down field edges and unhayed portions of fields that fit the OP's description of what he will be doing, any faster and there's too much volume of material for the bush hog to digest and it slips the clutch. Running a 10' cutter would take roughly an hour and a half at that speed if and only if there is not much extra time spent in turning around and maneuvering around obstacles. I question if somebody could run a 10' mower at that speed with slightly over 30 PTO HP, but for the sake of argument, let's assume it is true. The OP would be spending an extra $7-10k or so to save only about 2-3 hours per year in mowing time as he would save roughly one hour per mow and he mows three times per year. Also, let's not forget that he probably will only be spending somewhere between 6 and 7 1/2 hours in the entire year mowing if he had a 6' cutter in mowing those fields, it's not like he is going to be mowing for 200+ hours per year. I also bet once you figure in maintenance time (there are a lot more things to lubricate and keep tightened on a batwing vs. a single spindle cutter), he may actually spend less total time with a 6' cutter or so instead of a 10' batwing. It may be a different story if you are contract mowing a bunch of fields and thus want to finish as absolutely quickly as you can, and have a lot of total hours of work to do as you are mowing a lot of acres in total, but that is a very different scenario from what the OP stated. That is why I recommended a 5-6' single spindle mower for the OP.
As far as the comment for the 10' mower being more appropriate for 40-50 acres, if you mowed 50 acres with a 10' unit at the same speeds mentioned above, it would take you about 15 hours to do this. A 15' unit would get done in 10-11 hours. There is a little time savings there but again, it's not tremendous. A 20' unit, which is the largest commonly available and which would require a whole lot larger tractor, would save you only about two good days' of work per year if you cut everything three times per year. So again, the total number of acres you cut in a year is quite important and the more acres, the larger and more expensive of equipment that can be really justified.