The reel should balance in weight with the fly rod and make the whole outfit feel comfortable. Reels that are too heavy or too light are a distraction that can effect your casting and add to fatigue. Generally short rods balance better with lighter weight reels and longer rods with heavier ones. Big trout love to backlash "first-cousin-to-the-tomato-can" fly reels.
A backlash will end every fight in the trout's favor. Reels must be backlash-free at the lowest drag setting you will use. Disk drags are useful when using tippet sizes above the 4 pound test. However most disk type drags have too much inertial startup pressure at the lowest setting for fine tippets.
Click-pawl type drag systems usually have the lowest inertial startup pressures. Every reel should hold a minimum of 75 yards of backing. You won't need it for the average fish but you will need it for every fish of over 4 pounds.
