The large arbor fly reel is worth considering. They are becoming very popular for good reasons. The overall diameter of the reel and arbor (the arbor is the center of the spool where the backing is tied) is larger than a standard trout fly reel.
A common large arbor trout fly reel for a five-weight rod will weigh about 5 ounces and have a diameter of about 3 ½ inches.
The larger arbor increases the overall retrieval rate of the line. If the spool is almost empty a small arbor reel could have as little as 3 inches of line retrieved for every turn while a large arbor fly reel could have almost 9. This will make a big difference when you are trying to get the fish on the reel.
The fly fisher often has long lengths of fly line at his or her feet. This happens after casting and stripping in some of the fly line. After hooking a trout this line needs to get on the reel as soon as possible.
The line also comes off the large arbor reel in larger coils (called line memory), which has the advantage of fewer tangles.
Because the spool diameter is larger, the drag on these reels will be more consistent than a small arbor fly reel.
The drag on most large arbor trout reels tends to be smoother and will protect lighter tippets during a “fish fight.” The Orvis Battenkill and Ross Evolution are good examples of a large arbor trout fly reel.
If a fly reel manufacturer wants to increase the arbor size of a reel they must either increase the reel diameter or its width. Line tracking problems during line retrieval can happen if they try to keep the diameter small and only widen the reel. The challenge comes in finding the optimum ratio between width and diameter. Keep this in mind if the fly fishing reels you are looking at seem to be wider than normal.
The large arbor fly reel is worth considering, but a novice fly fisher should not need to think too much about these unless they find one at a really good price or they plan on trying their hand at steelhead fly fishing.
The only real disadvantage may be weight, but careful matching with the rod will minimize this.