Generally, what is the relationship between an organism's size and its biotic potential?

The answer is; the higher the size of the organism the lower its biotic potential

This is because larger organisms tend to have a lower reproductive rate those smaller organisms.  Larger organisms usually have one or two offspring in a year while small organisms. Biotic potential is the ability of a specie organism to double its population is a short time.

i think the miocene epoch spans the time from 23 million to 12 million years ago.