Can we tell how big a cloud is from the size of its shadow?
Don't we have to correct for the fact that the clouds are closer to the sun?

No, because the sun is actually much farther away than the clouds are

Here is an example -- a view through an airplane window.
Sometimes you will see sunlight playing on clouds or mist, so that you seem to see sunbeams diverging from each other, as in the first picture. The beams actually are all parallel to each other, and the divergence is the same trick that perspective plays on us, that makes the two sides of a straight road seem to converge towards a point on the horizon.

Push the "back" button