i have just achieved orbit in freespace. it took me 4 hours to find a moon that the engine could render. after scratching titan and tethys, i plucked mimas right out of saturn's orbit and stuck it into fred (this is the kind of power that gives me a woody). with only a 200km radius it fills up a good size chunk of space in the game, and gives you plenty of room to manuver. no atmosphere on this tiny rock. technically any planet or moon big enough to hold an atmosphere wont fit very well in the space allowed. thats not to say you cant fake it. titan would have worked great if it would have been less than 750 km in radius. i wouldnt go over a radius 500 though if you want room to manuver. 500 would give you about 100 km of atmo and 50 km of orbit space before you get accused of leaving the mission area and self destruced. enough if somone wants to make a hellbender mod.
entry into a gas giand as i had though is not possible, however, it can still be done in planar mode and using a skybox. just split the mission. gas giants tend to have pretty diffuse upper atmosphere with quite a far reach. you can use a skybox just fine, just set yor base level really low. then end the mission when your y position passes a certain threshold, then load another mission in a nebula, set the atmospheric layers a little dense, flying at a good aircraft speed. finish the mission and maybereach escape velocity.
