When I designed my first reels, I intended to accomodate 5 weight lines. I set the spool capacity from measurements of some other reel, now forgotten. So yesterday I made some measurements on the line that I installed on the prototype reel (same spool size as the later group). This line consists of 162 feet (54 yards) of 20# backing and a 90 foot (30 yard) WF4F.

My reel spools have OD = 2.6 inches, ID = 0.5 inches, and spacing between spool plates of 0.83 inches. So the volume available for line is 4.24 cubic inches.
i.e., volume = pi/4 * (OD*OD – ID*ID) * spacing.

When I had wound up the 162 feet of backing, the diameter was built up to 1.3 inches. So the backing took up 0.94 cubic inches. Then winding on the line brought the diameter up to 2.0 inches. The line volume was 1.51 cubic inches.

At 2.0 inch diameter, there is plenty of room still left on the 2.6 inch diameter spool.

If the line had been 5 weight instead of 4, what would the wound-up diameter be? Typical weights of fly lines are:
3 wt 100 grain
4 wt 120 grain
5 wt 140 grain
6 wt 160 grain
By proportions, a 5 weight line should occupy 1.76 cubic inches, and on my reel the built-up diameter (with same backing) would be 2.1 inches. This would still fit comfortably on the reel.

Update 16 Feb 2012: 54 yards of this backing filled 0.94 cubic inch, so 100 yards would fill 1.74 cubic inches. I don’t really know what brand backing this was; it was something that I stripped off an overfilled reel some years ago. Not really sure now that it wasn’t 30# instead of 20#. Just bought a new 100 yard spool of Scientic Anglers 20# backing, and it fills only 1.10 cubic inch.