So we now have 3 strong hints at the roughly 120cm (4ft) size, including the head. Assuming this is correct, Equestria has insanely high gravity - more than the gravity on Jupiter!
There's a video investigating that which I tried to debunk but ended up mostly confirming, see heresee here for both the original video and my (hopefully more credible) calculations.
So we've calculated Equestrian gravity assuming 4ft size; but what if we try the reverse - assume Earth-like gravity (g=9.8m/s^2) and try to calculate size?
Using my previous gravity researchprevious gravity research and assuming Earth-like gravity, Spike's spectacular jump into the lava was merely 3 meters high! The highest point of the jump measures 16 Spikes, so one Spike is 300/16 = ~18cm, or 7". That puts mane 6 at just 25 cm or at about 10" tall, including the head.
Unfortunately I can't tell if it's the ponies that are really small or if it's the gravity in their world is so insanely enormous. But either way, the "human transported to Equestria" fanfics are not credible: the human would either be crushed by the enormous gravity, or turn out to be just too damn huge compared to the ponies.
If the latter is true, perhaps the human should try to hang out with dragons instead? They're more their size!