"LBR, are you saying that Bohning Signature nocks are tighter than their Classic nocks?"
Yep--at least they have a harder snap, which is what will affect your arrow flight/performance.
"Are there standards for nock sizes?"
I think there's 3 or so "standard" sizes, maybe more, and some brands change from time to time.
There's two different sizes of G nocks (large and small--couldn't quote the diameter), at least two fits for Easton uni-nocks (3-D and Super nock), Bohnning Classics, Bohnning Signiatures, Mercury, Nirk, Z, H, X.......heck, I can't keep up with all of them! I keep a box of assorted nocks on hand so I can fit the serving to the nock on a particular string, but sometimes I have to ask the customer to send me one of theirs when I'm not sure or don't have one on hand. Some folks like a tight fit, some like it loose.......erring to the small side and building it up with a couple of wraps of dental floss is what I'd do.
Chad