Limitations and access issues of 40' or better?

If you're asking about size, you will not have issues with a 40'. We full-timed in one for 10 of our 16 years. We boondocked down miles of gravel roads and primarily used public parks - national, national forest, state parks, county and city parks. We also traveled to Alaska. We've never been turned down because of size. Of course, we did our research before going to certain parks.