I don't know about the length of the short flap. However, I'm just under 5'2" with short legs, and I ride in a regular flap. Also, the different models have different size flaps. The Stabilizer has a larger and longer flap than the Innovation.
I'm 5'5" but have somewhat short legs for my height (inseam is 28" or so). I can ride in the regular flap on both the Stabilizer and Innovation okay, but I do have a short flap on the Stabilizer that I had made for one horse. Since I was going custom, I figured I might as well do that, and if given the choice, I prefer the short flap. I'll go measure and post that when I'm able to later today.
I also had an Innovation for awhile that I bought as a demo to fit a particular horse. It had an extra forward flap, and I hated it! So the forwardness of the flap really made more of a difference to me than the size.
The short flap on the innovation is 13" which is 1" shorter than the regular flap.
I am 5'5" and ride in the short flap - I found a used innovation that was the right seat size for me and tree size for my horse at an excellent price in almost new condition - with a short flap. I purchased it and have never noticed the flap to be too short.