Generally its slightly smaller than mono, but it collapses and lays flat on the reell, so you get a lot more dacron than mono on a reel. IMHO its time is past and you're better off using the much thinner super braids.
I would use it on a plastic spool reel as backing as it makes for a good cushion and would lessen the chance of a spool imploding under pressure. The penn 710 greenie and squidder plastic spool reels come to mind.