As long as you don't space your wheels wider than your mower deck and loader bucket, any size will work.
As far as the 54 inch mower on my B2601 is concerned, the wheels on the mower are just ahead and further out of the back wheels, so looks like anyway, that 2 inch spacers would put the rear tractor wheel/tire edges around a inch or so behind the mower tires. Hopefully that's not to close. I am contacting Bo-Tec now on spacer price and thinking about buying the $335.+ $30. shipping ones on e-bay, but hoping to find some cheaper ones that come with studs. What kind of spacers is that in the picture you posted?