Bigger is better for close-ups, but for long shots, if the ship is far away from the camera, you want a smaller miniature, since a bluescreen stage is only so big and the camera can only move back so far. Here's ILM's model shop supervisor Jeff Mann from "Creating Special Effects for 'STAR TREK: The Next Generation" in Starlog
's ST:TNG magazine issue 2, p. 28:
They asked us to build a six-foot Enterprise and a two-foot Enterprise; presumably, one for close-up beauty shots and one for long shots.
EDIT: Oops, just beaten to it.