Welcome aboard - and your KUbota will handle a 5 footer easy and possibly a 6 footer DEPENDING on speed and how high and what type of grass you are cutting.
Check out the link below. I would keep it to 5' or less for your L2500. I have a 5' Landpride mower on my L3901 and it can put quite a load on the tractor if what you are cutting is thick and you have any hills to deal with.