I’d go by the ETT, the effective top tube, that gives you an idea how long the bike is horizontally when you’re sat on it. Then you have a bit of freedom to add / subtrack 10mm or what you like on the stem.
Stack will adjust how tall the front end feels, and how much you’ll be pointing arse up and head down. You’ll be able to adjust that with spacers on the steerer, but you wont be able to adjust a head tube size…. too tall on a 29er, and you’ll end up flipping the stem, too short on a 26 and you’ll have loads of spacers under the stem and look a bit.. well… odd.
🙂
ETT for me is the most important for an *idea* of fit, then I’d be looking at HA, SA for how the bike will pitch you forward or back, but like everyone will say, you’ll need to ride one to really know.