lift the deck up and measure the distance from the anti-scalp wheels to the rear tires on the horizontal plane.
In my case with my BX 2200, I think 1.25 inch spacers was the limit. Granted, I have the 60 inch deck. But in any case, when I raise my deck all the way up one side rubs on the tire a bit. When cutting grass it’s no problem.
Nothing beats an empirical measurement.
And the next question is, where do you buy your spacers? I bought mine off of eBay, or Amazon, don’t remember which. They actually were for a jeep, but they fit the BX perfectly. Also worked on my son-in-law‘s BX 23S. Much cheaper than other sources.