Taking an approximate measurement for the diameter of the Earth, and assuming a hemispherical dome as it's the most likely shape to form, 63000km, more or less, centred at a point just off the north pole.
In lieu of an FE answer, have a REer's educated guess as to what the model would contain. I'm assuming the dome does touch the sides of the Earth rather than being somehow floating, but that wouldn't be the weirdest part of FET I've heard if it somehow was.
The highest point north of the equator would be the greatest height. The highest point south would be at the equator itself, (or technically arbitrarily close), and would be just over 17300km.
Then again, that would be under a few educated assumptions. Without any better elaborations from FEers, looks like the best we have.