The editor can handle files smaller 2GB, that should be enough. If you want to include files, write seperate .gbas files (on the right pane, open the "files" tab and right-click on "new file" or something. The edit the file, save it, make sure it's in the file-list.
In the additional files you write, do not have code outside of FUNCTION or SUB - that's only allowed in the main program file.
Extension file names should be .gbas as well.
Thank you for the damabed link info. I'll fix that with the next update.