Determining max valve size.....
 

Do you just measure across the relief in the piston?? Also does the depth have to be considered into determining the max valve size??

Valve size isn't dependant upon the size of the eyebrow in the piston (rather it is the other way around, the eyebrow should be dependant on valve size, but most pistons have large reliefs cut into them so that they can support a variety of configurations).

I believe valve size is dependant on the bowl size behind the valve and how well it flows.