Well, Kaz himself claimed that they're shooting for each of the creation environments in the course maker to be 100x100 kilometers. He said that it's looking good, but if there's an unforeseen problem, he can guarantee that they'll at least between 25x25 and 50x50 kilometers. To put that in perspective, the Nurburgring would fit in a 100x100 kilometer area something like 15 times; though memory limitations would never allow you to put that much track down in one build. They also said that you will indeed have much more control than GT5, though they haven't said how yet. Each of the creation areas will double as photo landscapes, and will be more sprawling and diverse than previously.
I'll see if I can find the link to one of the videos with this info, as a source.