Figuring out the capacity of a fly reel is more fun than doing your tax return, but not necessarily easier. With experience, it gets a little easier. Find the manufacturer's recomendation, though they're not always accurate, or helpful (Pflueger used to give the capacity for their reels with double taper lines - not much help with a 1498). Outside diameter is just a start. Some reels have the same diameter, but with a wide spool model that holds considerably more than the standard model.
Design can make a huge difference, especially medium- and large arbor reels. An extreme "Ferris wheel" large arbor has lots of space within its spindly frame that holds nothing but air.