I've never heard of Blue Ox. I've always gone with Reese or Curt hitches.
Hitch balls come in 1-7/8, 2 and 2-5/16 inches. Very rare are 3 inch hitch balls. My canoe trailer is 1-7/8ths, the campers I've had were 2 inch. Saw my first 2-5/16 hitch ball for a flat bed trailer at a company I used to work for.