would you rather have a frame one size too large or one size too small?
I've always tended to feel more comfortable and be capable of riding in the saddle longer on a frame that (according to conventional wisdom) is deemed "too small."
Having relatively short legs and being long-waisted, but preferring a more-upright riding posture, I've generally found a bike that's a couple of sizes smaller (based on ST and TT measurements) works better, and can be made to fit better. But, that's me.
I'd be a stellar candidate for a custom geometry. Much shorter reach, greater stack, an appropriate orientation (of course) of saddle position to cranks.
Current bike: a 15" Trek 970 MTB, with riser stem, swept/riser bars, 165mm crank arms (probably should be 160mm) ... yet I'm ~5'9" with a longer torso and shorter-than-usual inseam. Most typical bikes have me stretched out like a 2x12, yet that's never been how I've ridden. So long as I get the saddle to crank position about right, on such a seemingly-puny bike (for my stature) I can fiddle with stem+bars to get it comfortable; whereas I can't achieve this on a typical larger, "right-sized" frame.
Of course, everyone's limb lengths, riding posture and fitness differs. But the above works well enough for me.