the shims are 25mm diameter.
The thickness in our motorcycles will range from (about) 2.60mm to 2.85mm (I said generally).
The 1stgen, 2nd gen, (inc RSTD) and 1st gen Vmax use the same shims, as do about 50 other makes of bikes and 1970-1990 Toyota 4cyl engines (the most widely produced engine ever made, with heads designed by Yamaha). The Toyota shims tend to be slightly thicker on the average than Yamaha MC shims. I bought some used Toyota shims and only about 50% of them were in the above thickness range.
shims removed from engines will be found in any thickness, in 0.01mm increments.
The most common commercial replacement shims are in 0.05mm increments
Triumph MC does sell 25mm shims in 0.025mm increments, but are more difficult to find.
I found a good usable variety of used shims from a couple of parts bikes, but as FF said, ymmv.