Flyfishing reels are very specialized reels. Fly reels hold what is known as backing line, which is tied to your actual fly line.
The "Fly" line is weighted, which allows you to get a good distance casting. Fly line comes in floating and sinking, and just be used according to what kind of fly that you are using.
The Reels are sized by weight, the different weights, similarly to the other reels, hold different lengths of different strength lines.
As a generality, the following could be said regarding reel size to the line size:
4/5 Weight - 100 yards of 20lb line.
5/6 Weight - 150 yards of 20lb line.
7/8 Weight - 200 yards of 20lb line.
8/9 Weight - 225 yards of 20lb line.
10/11 Weight - 400 yards of 20lb line.