Snarfed from another web site:
Quite simple, really.Size of Lunar Module. Let's be really generous and say 10m square. Distance between Hubble and Moon. About 350, 000km. This works out as an visual angle of (10m)/(3.5 x 10^8m) * (180/PI) = 1.6 x 10^-6 degrees = 6 milliarcseconds. The WFPC2 'telescope' on Hubble has the following resolution: 800x800 pixels of a 35 arcseconds field of view with a pixel scale of 46 milliarcseconds. Actually resolution in practice is a little below this.So what does this all mean? Well, roughly speaking, it means that the LM would have to be 15 times larger before it would even cause a dot on a Hubble picture.
The wavelength of visible light is around 550x10^-9m (i.e. very very small). The diameter of Hubble's mirror is 2.4m. Highest ever physically possible resolution = 1.4 x 550 x 10^-9 /2.4 m = 3.2 x 10^-7 radians At a distance of 350,000km this works out as about 124 metres. As Ed says, roughly the size of a football field.