I've found lots of discussions on how to get an upgrade. But is there a best approach to refusing one?

This weekend I reserved an intermediate (e.g. Corrola, Focus) from Enterprise. The only car they had available was a Jeep Compass. It was, for my trip, too big and had a crummy ride and lousy gas mileage. But it was a "free" upgrade because the suburban location didn't have any others available. It was only a short rental, so I took it.

But in the future, can I refuse it? Are the only options limited to their lot? Can I request one at the same price from e.g. their airport location? Can they get me one from a different agency, either National or another?