how much information can beam of light can have in unit area? Can it have entire Earth's image? Can one have a big lense in distant Galaxy where he can see everything (like humans walking and so on..) that happening in Earth? (Though there is a delay that's not important)
 A: You would need ridiculous angular resolution to do such a thing. In astronomy a very well known formula to find the angular resolution of a telescope is given by the Rayleigh criterion which says that the angular resolution is given by $$\theta = 1.22\frac{\lambda}{D} $$ where $D$ is the aperture diameter of the lens, $\lambda$ is the wavelenght of the measure light and $\theta$ is the angular resolution in radians. To make matter worse there're so many factors that degrade the capacity of a given telescope to resolve two point-like light sources: seeing, differences in atmospheric index of refraction, conduction currents in the atmosphere due to heat, and so on.
One way that astrphysicists try to get better angular resolution is to use an array of telescopes, like the ALMA telescope in Chile. Using an array of telescopes makes a greater resolution since now $$\theta \propto \frac{\lambda}{B}$$  where $B$ is the distance between two telescopes.
But in any case, the light of distant objects could be so faint that it would not be feasible to look at them with any technology, especially planets since they do not emit light by their own.  
