A 24" square hearth. That's making my mouth water
But $1600, that is a pretty penny. It's too bad the inexpensive Chinese countertop ovens that a few members here own don't come in 24". I think 18" is the norm.
As the diameter decreases, I'm guessing oven spring might improve a bit, but when I talk about less dough, I'm referring to a lower thickness factor, which you're mastering right now.
Jay, try 14". I really can't see how someone with the ability to stretch an 18" pie that flawlessly, can't launch a 14" pie on a 17" stone. I'm curious, how close is your stone to the back wall and the front door of the oven? If it's a small gap (less than 1/2"), then don't be afraid to launch the pie into the back wall and have it touch. If it's touching/almost touching, then you don't have to worry about it hanging off the edge on the front.
As far as the amount of dough to use for a 14" pie... that actually gets tricky. As you go smaller, the size of the rim has to stay pretty much the same to avoid boilovers. Since the rim size is static, the dough doesn't decrease proportionately (nor for that matter, does cheese or sauce quantities). It's really a J curve, not a straight line. 520g for 18" looks great. I use 470g for 16". I can't make any guarantees, but I would try something around 430 for 14"