How do I know I have enough room to back down driveway?

You are right to be concerned, as this is not the best backing situation, especially for a beginner. Can it be done with the size truck/trailer mentioned? I think so, but not easily. Then you may have traffic, or cars parked on the street, in the way for maneuvering. Lots of RVers deal with similar issues, just depends on your patience/tolerance for manuevering, and a helpful spotter.

Jerry