How much smaller is the diffusivity of a molecule in the retina as opposed to the vitreous of the eye?
Unfortunately, I don't have a good, simple answer for you. The reason is that the answer may depend significantly on details of the macromolecule more than its size - like how strongly it binds to the environment.

In addition, within an environment that is more elastic than viscous, and with a very complex shape, you might expect to get various sorts of anomalous diffusion, rather than a single diffusion coefficient. 

Transport in tightly confined biological spaces is complex, and still pretty controversial. A paper I found useful as a simple introduction to some of this:
Morphogen Transport - http://dev.biologists.org/content/140/8/1621