I'm not sure there's such thing as an optimal structure, it depends on the exact place you want to build the track, but I know which track is the best I drove on -- it's the one in the infield of the Spa-Francorchamps Formula 1 track. You can see the layout here. I think it's about 2 acres in size.
There are, though, some things that I think you really want to include in a track: a long start-finish straight, at least one fast corner, at least two challenging, technical corners (a tightening corner, for example, maybe even off-camber), a hairpin or two (most tracks have too many of them), a and chicane (preferably one with an alternative bypass).