Unfortunately as far as I am aware at least - you cant copy FPSC level content to the clipboard to paste into a separate or new level. Its something I suggested way back for to be considered for inclusion. It would probably be seen as of very low priority so I dont know if you will ever see it happen.
You can make your map as large as you want within the world size and FPSC engine capability limits. In large complex levels you may need do a lot of work in optimisation of levels to sustain suitable gameplay speeds. As to acceptability of load times - large complex levels will by any measure of professional AAA title game standards have very long load times - in the order of some minutes. Its a matter of opinion as to whether or not they are excessive - they are the way they are - and your game player has to put up with it if you want to make decently complex levels and they want to play them.
Personally I would not worry in the slightest if that were the case although of course would prefer it if it were not so. If any game is good its worth waiting for and if its not then fast load times wont matter - players would be more likely to be a bit miffed if they have to wait for the long download time of a large game or game levels which FPSC outputs only to find they did not like it. If its good they wont worry about the load time. Make the screen shots, demo, intro and load screens and audio good and they will be too excited waiting to play to worry about the load.
"I am and forever will be your friend"