I find some amount of different advice on primer sizes is the reason I ask.

There should be no question on primer size. Small won't stay in a large pocket and large will be destroyed trying to get it in a small pocket. I use magnum primers when the powder being used requires it, otherwise a regular primer is used. I don't shoot any of the high pressure gaming rounds so using s small rifle in a pistol case is out for me. I would NEVER use a pistol primer in any rifle case.