star - What is the scale of things you can see with gravitational lensing?

You are right that the stars seen on the sky are within the Milky Way. Only with a large telescope is it possible to resolve individual stars in other galaxies, and only for the nearest ones.

I don't know which sources you refer to, by I think perhaps you are confusing the different types of gravitational lensing. I cannot explain them better than the excellent review by astromax, but briefly, there are three types:

1. Strong lensing, where are foreground galaxy cluster (i.e. a group of $sim$100–1000 galaxies) magnify and severely distort background galaxies,




2. Weak lensing, where clusters or individual galaxies distort the shapes of many background galaxies on the percent scale, which can only be seen statistically, and




3. Microlensing, where a single object inside the Milky Way happen to pass in front of another single object, also inside the Milky Way. These objects are usually stars, and do not distort the images of the background objects, but merely increase the flux for a while. This effect has been used to find exoplanets.

Whereas type 1 and 2 are more or less static in a human lifetime, type 3 is an event that happens once only for a given set of stars (as seen from Earth).

Scales

Strong and weak lensing happen on very large scales, from a few hundred million lightyears, up to the order of the size of the observable Universe (e.g. Wong et al. 2014). While the lenses themselves are cluster of galaxies, and thus a few to $sim10$ megaparsec across, the lenses object are typically individual galaxies. Gravitational lensing is most efficient when the lens is halfway between us and the background source.

Occurring in the Milky Way, microlensing, on the other hand, happen on the scale of a few kiloparsec, again with us-lens distance being of the same order as the lens-background object distance (see e.g. this Wikipedia list).