I am just about to use a 1200w 12v Pc power supply to power a falcon f48. Should I use a fuse between the power supply and the falcon and what size? Or just rely on the fuse on the board?
If it's just an F48, then that's massive overkill. The board itself only needs about 2-3A at 5V, and less at 12V.
A 5A fuse should be more than sufficient inline; it should be sized based on the wire gauge you're using to connect it to the PSU.
The fuses that are on the board protect pixel outputs on the inbuilt smart receiver (if it's a v4); not the actual controller itself.