That's about ~250x the JWST. Given that we're talking about a society that has developed far enough to be sending people to Mars and terraforming the landscape, I don't think that 8 doublings would be an unreasonable multiplier of current capability. Still a flagship, many-decade mission though.
The James Webb mirror is a rigid, astonishingly precise focusing element. A solar energy mirror could instead be merely approximately parabolic, made of a foil instead of cryogenic, made of metalized kapton instead of gold-plated beryllium, would forego focusing elements, and so on.
However, the thought of a telescope-quality mirror 250x the size of the JWST is pretty amazing :)
