You can make any model for the bit you land at. If you want to make an area you can fly around as a planet's 'surface', it is possible, you just need a large CMP model. My advice to you is to attempt to make it and then you'll need to add it in as a new system.
This can then be linked to 'docking' at a planet easily enough.
As to limits, the limit is dependant on each persons system. I have witnessed first hand objects in Freelancer that are 200k polygons.