(edit2: Actually I realized I really meant "fixed" ). Sometimes I want to get in to see something closer but the thing is just shrinking as I'm trying to get a better look.
However, what I think is the best is to make the scaling very subtle - certainly nowhere near proportional to the zoom distance. So when very far out stuff would seem smaller yet nice and visible, and when zoomed in close stuff would seem somewhat larger - but in reality when you're zoomed in closer while the model is larger on your screen, it's absolute scale is somewhat smaller than when you're zoomed out.
Am I getting the point across or is that confusing?