I find this thread super interesting. And if you read anything by Paul Aston or the Mojo people I'm sure you do to.
I was on a large Giant Trance. Tested the small (then) Stumpy Evo, the S2 size and loved it. Got back on the trance and it felt too small.
Bought the S3 Evo, in 27.5 because it was the longest bike I could easily get at the time. I think a 490mm reach?
I later got very lucky and got a Kenevo at a very affordable price and went for the largest, the S5 with 520mm reach. Kenevo was amazing, so much so that getting back on the Evo felt like getting on a BMX. Ended up selling the Kenevo and Evo and bought a Geometron G16 (from the forums here) with 520mm reach and instantly loved it.
Had been set up stupidly slack, approx 60 degree HA, but even with that it still climbed great and was a monster on the downs. I did heaps of experimentation with different shocks, different flip chip positions, different wheels front and rear and genuinely they were all good, just different. For me the extra length made feel like I wasn't cramped and could stay "inside" the wheels, while all the other adjustments just changed the feel.
In fact I liked it so much that I ponied up to go even longer and bought a G1 frame with a 535mm reach and it is fantastic. I have had no issues with that size at all, and it's a very big bike.
For me the reach seems to be the most important number, because I don't need to have my seat jammed all the way, which automatically makes climbing easier. A steep seat angle, long reach and a slack end all feel great to me. I'm also convinced that longer chainstays are better. My theory is that the story CS makes the bike more out of control, which makes it feel more "playful".
Lastly, I'm convinced that a higher BB is better. The Evo was a downhill weapon, but way too low and pedal strikes were pretty constant. A higher BB and shorter cranks have had no downsides for me at all.
Now I'm not a great rider, by any means. I'm too heavy, I'm too wussy and I don't have great skills. For me, a bike that looks after me is better, while a high skill rider might be quite comfortable being constantly a little bit out of control.
I'm 188cm with a normal wingspan, but very long legs, and I can't really see myself on a bike shorter than 520mm. At least, not without a very good reason.
My feeling is that someone who is a fair bit shorter than me would still be fine with that reach.
And on a related note, I have a 520mm reach G16 frame for sale in fantastic condition, people just need to look past the long reach to realise what a sweet bike it is! I've upgraded to longer, not necessarily better.