I had similar thoughts recently regarding Aeris sizing, and ummed and ahhed a lot!
I am 178cm (5’10”), with an inside leg of 32”, and arms perhaps slightly longer than average for my height. For my old bike, a Whyte T130, which had a 609mm top tube (no idea on reach sorry) and 55mm stem, that was about right. Medium with its 594mm toptube and 1132 wheelbase was too small. For most ‘’traditional’’ geo bikes, at 5’10” – 5’11” you are generally probably better off on a Large. This is particularly the case for the ‘’smaller’’ brands such as Santa Cruz.
On the Aeris, I went in with the same sort of attitude. I test rode both the medium and the large, both with a 35mm stem. On first test ride (while both felt great) I felt that the Large was perhaps a better fit, as it was that bit more roomy and stable, especially when riding around the car park. However, I certainly wouldn’t want to be any more stretched. i.e. I was at the limit, even with the 35mm stem, and I wouldn’t want to run a shorter stem like that for general trail riding.
Still undecided, I had another test ride, and after this further time on the bikes, I actually decided that Medium was better. It was still stable and roomy (thanks to its 1187mm wheelbase and 448 reach) but was just that bit more nimble and playful through twisty sections, whereas on the large I tended to run wide a bit more on corners. In addition, while I could just about run a 150mm dropper post on the large, it would be at the limit really, so the medium gives the opportunity to really slam the saddle if and when required! Likewise, the standover on the medium is that bit better too.
Ultimately, I could ride either size (and there really isn’t much in it to be honest). If I was only riding flat out, open tracks at v high speeds, trying to get air at every opportunity, the Large would probably be better. For more twisty trails, the medium would be better. For general, all round trail centres, bike park Wales and everything in between, the medium seemed to be the better overall fit. I therefore went for a medium, albeit with a 40mm stem (but could have run a 35mm stem). And as above, it is still long for a medium!
Other things of note – Olly Forster of Bikeradar rides a medium Giant Reign and believes it is a good fit. It’s pretty identical in terms of top tube, reach and wheelbase to the aeris, and he rode a large Kona Process previously. Likewise, Tomas Kuptys, a Bird rider, rides a Large aeris, but he (appears) to be around 6’3”.
When I spoke to Bird in more detail about this, they said that generally people around 5’10” go for a medium.
I may have confused you further, but hope it helps!