On my Surface Pro 3, there's a built-in program called "On-Screen Keyboard" that pops up much smaller keyboard with all functions. Not only the Fn key but also PgUp, PgDn, etc.
It also can be moved around on the screen easily to position it where you want.
Just go to Start/Search an enter "keyboard" You should find "On-Screen Keyboard" which you can pin to the Start menu and/or the Taskbar.
It's a little harder to type on because the keys are smaller, but it's really handier to use.