A scratch built Cherry Valley 0-6-0

[Sumpter250]

do you know if there is a formula to figure out the sizes by wheel diameter or piston stroke or something like that. I hope you could help.

Pretty much, all the measurements for valve gear, are based on the stroke of the pistons. I found a CNJ 4-6-0 in MR's cyclopedia Vol I, but no Mammoths (4-8-0).
Typically, the distance from the center of the driver, to the center of the crank pin is equal to half the working stroke of the piston ( overall length of the cylinder, minus the steam port openings ) The eccentric crank, seems to be approximately 1"-2" larger, center to center.
The crosshead is dependent on the diameter of the cylinder, as there has to be space on the back of the cylinder to attach the crosshead guides.
The maximum travel of the valve (and, therefore, the valve gear) would closely match the "working stroke" of the piston.
If you have a photo, and can determine the diameter of the drivers, you should be able to determine the "scale of the photo", and then the relative sizes of the rest of the parts. If you are using the drivers shown in the photo (looks like they're 45" dia), use them as the means of determining relative size, specifically, the distance from the center of the driver, to the center of the crank pin.
Hope that helps,
Pete