Piston/Casing Table Sometimes, it is helpful to know the original size of pistons/casings.
https://www.horn-u-copia.net/Reference/casing%20table.html
Assuming the instrument you are working on is on this list, it is a good place to start. You then know what the piston was when new. And a rough idea of what the casing was when new.
From there on it is a judgment call on how much larger you want the piston to be.
There must be some clearance between sliding parts. For example, if the pistons are 0.6650, the cylinders might be 0.6653
The chart should only be a starting point. It tells you what the horn started out at when new. It doesn't account for wear on a well-used instrument. You will want to verify what your actual casing measures are and work back from there.
The diameters may vary per cylinder.