The models are highly tuned to a specific set of parameters - this is what makes them fast. Going outside of those parameters would make them misbehave and generate increasingly less realistic planets. We could probably allow planets slightly larger than Earth (if I had to *guess* maybe up to 1.25 or 1.5 of Earths radius) but I haven't done testing to verify whether it works.
Technically speaking this means we should also lock the radius more strictly from"below" - this will be done in the next version (probably down to Mars sized planets but I haven't decided yet)