Oh, and for another reference point. I've recently gotten a 1993-ish MTB frame that could be either a 17 or 18" depending on how they measured (center of top tube or top of seatclamp).

It has an ETT of 56.5cm.

As to the 650B they can be nice as they give you higher gearing than 26" with the same width tires on. Limited tire selection is a bummer though.

If you need higher gearing many steel frames from back in the day can handle 50-53 tooth big rings