I would replace their board with a RAMPS board first, as that will allow you to really mess with the firmware and not worry about screwing up your original board, just put it in a box safe somewhere, rebuild your machine using the firmware so to speak, the machine is built you just need to re-wire it accordingly and it is pretty easy, this would then allow you to choose the size of the hotbed. The FF use reasonably custom mainboard, and personally the tutorials for RAMPS and repraps far exceed the FF mainboard tutorials, so if you want a smooth ride, i'd do the above.

Just be aware,, the bigger the hotbed and bigger heating element, the more current it will draw, so just make sure your power supply is rated to draw that many amps + what you are already running. Some of the power supplies are at the bare minimum needed to run the printer, so adding extra things can also require a new power supply. The flashforge should be fine to add a 20cmx20cm hotbed to no worries without changing supplies.

something like this is all you need really to upgrade it (just the RAMPS, drivers and mega board, the endstops are not required of course)

http://www.ebay.com.au/itm/3D-Printe...item1c3c4b1f46

and of course your heatbed