I used to have a B5100, the owner's manual covered the B6100 also. I recall the recommended sizes of brush mowers 36" for the B5100 and 42" for the B6100. 2' tall weeds will demand some power, you might get away with the 48" if you don't make full width passes.
I believe it will handle a 48" light duty hog, but your gonna need some weight added to the front and, as always, careful use. Avoid cutting too low or under wet conditions. My 6100 doesn't have a temperature gauge, but its a tough little machine and can work itself into the red zone if you let it.