The spatial resolution of a telescope with a diameter D and using light of wavelength l (should be lambda) for an object at distance d is given approximately by ld/D. Equivalently, if you want to resolve something of size R then you need a telescope of diameter ld/R. So, for instance if you wanted to resolve a metre on the moon (distance ~ 4E8m) in green light (wavelength ~ 5E-7m) then you need a telescope with diameter of about 200m (this is why we can't see the Apollo landing sites from Earth).
If you wanted to resolve a thousand km (10^6m) object at Proxima Centuri (about 4.28ly or 4E16m), again in green light, you'd need a telescope with a diameter of about 20,000m (this seems too small to me: I'm worried that I've made a mistake).
Biting the hand that feeds IT
