This "just format it..." is where I've still been lost for the last 2 hours.
I've got a similar project where I want to take a 500GB HDD image that's currently on a 1TB drive and make use of the blank space. I've got everything on F copied off fine, I just have no idea how to actually wipe and re-partition a single partition. F can go away and be reborn as a new biggerer F partition, FATXplorer just doesn't do this. Or at least I don't see a way to.
I've been looking at Chimp and XBPartitioner, seen those both mentioned.