Sizing from 29er FS to 26 HT
 

Would a person generally ride a smaller 29er FS frame than a 26 HT? I know all manufacturers have their own little quirks and nothing is truly standard, but is there some kind of rule of thumb? As long as I can ballpark it, I have no problems with changing stems/ seatposts.

(particularly, 18" HT to a 16.5" Rocky Mountain FS29r)

I have both 26 and 29, both medium in the specialized. I say stick with the Effective Top Tube measurement.

Fit is a person & bike specific thing - but, both I and a buddy dropped a size going to a 29er, (like 19.5 to 18). No regrets from either of us. I had a chance to ride a 29er in "my size" for awhile before I built one up, no joy there.