re: Question fro R.L. WRT Extrasolar Lensing and Planet Finding
All you detect is Mass. There is no difference between the signature of a small barren planet and a Borg hive. No atmosphere, no imaging, no spectrum (or rather there is, but it is of the background star).
Size limit is based on the ratio of star to planet in the lensing system. I'm not fully up to date with the method but I beleive it becomes marginal at around Earth size planet (or at least the mass ratio of Earth to our Sun).
Do a Google on it, you should find some useful info...