Shower Valve / Diverter pipe thread size
(USA - likely different in places that use other thread forms, of course) The valve is usually 1/2" FIPT, so the PEX adapter you'd need would be a 1/2" MIPT (and at the showerhead end you'll want a PEX - 1/2" FIPT Drop Ear Elbow so you can affix the shower arm attachment to the framing.)

Of course, you could actually buy a particular shower valve and know for sure.