it depends entirely on how many acres you are spraying , How many trips you want to make back and forth refilling the sprayer, With a 3pt sprayer i would get at least a 60 gal unit, that will allow you to spray 5 or 6 acres at a time.
I have a 55 gal 3 pt sprayer, and get about 2½ acres of coverage from a tankful. (20 gal/acre) With your size tractor, I would think you could go as large as you want. I only have 38 horses, and wish I had a 80+ gal tank.