Currently Earth’s size is defined properly in src/utils/Earth.cc as:
const double GPlatesUtils::Earth::EQUATORIAL_RADIUS_KMS = 6378.137;
const double GPlatesUtils::Earth::POLAR_RADIUS_KMS = 6356.7523142;
const double GplatesUtils::Earth::MEAN_RADIUS_KMS = 6371.002;
However, for working on other planets, the measure tool supports modifying only the equatorial radius. For proper calculations, being able to change both the equatorial and polar radius would be highly desirable.
It would also be nice to replace the string “Earth radius” here with “Planet radius” and support proj4 descriptions. For example, PostGIS comes with the proj4 projections for several planets including Mars.