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Again, I would not worry about that. Coticules are pretty hard and wear very slowly compared to synthetic stones in my experience. The wear would be somewhat uneven regardless because you would...

I would not worry about cutting it down (not sure what tools you would need to do that properly), it should work fine as is. If you don't want to use the whole stone then just stop your stroke early.

If that is your goal then the size that you mentioned should work fine. My "go to" touch up stone is a 1x5 inch Escher which translates to about 25mm x 120mm. Works great in the hand and would be...