When selecting the proper fly rod and reel combo, one of the most obvious things folks check is the reel's line weight rating. It's pretty simple, right? If buying a 5 weight rod, then you'll want something like a 4/5 or 5/6 reel. Need a saltwater 8 weight? A 7/8 or 8/9 reel will do nicely.
It only seems to make sense to first match the numbers accordingly, then you can go on to look at other factors like the reel's physical weight, drag system, arbor diameter, and other features. But, what if I told you that you don't have to match the line weight ratings of the reel and rod together exactly? This might not be a mind-blowing revelation to some, but up-sizing your reel can offer benefits without reducing fishability in the least.
When I talk about upsizing a reel, I don't mean go out and hang an 11/12 reel on a 6-weight rod, but moving up a size or so is totally realistic. Example: Using a 7/8 reel on a 6-weight. Two downsides here are that moving up to the slightly larger reel might initially cost more and be heavier to some degree, but if you shop your reels carefully both of these potential negatives can be of little to no issue. So, what about the positives?
More Backing: As long as you are comparing reels within the same model family, the larger of the two will always hold more backing than the other. For many anglers it's probably the least of their concerns, but for those that frequently tangle with long-running species, it can be a nice added confidence boost. The larger spool size is also a big help if you plan to up-line since the thicker fly line takes up more space on the spool. The bigger spool of the larger reel gives you extra space for that line yet still allows for a solid amount of backing underneath.
Faster Retrieve: A bigger reel means a bigger overall spool diameter, which of course equates to a faster retrieve. Granted, it might not be an insane difference, but any little bit of cranking speed helps. This is something all fly anglers should appreciate whether you're after rainbows or tarpon.
Multiple Rods: Let's say you want to buy 6 and 8-weight rods. If you don't think you'll ever be needing to have both rigged and ready at the same time (which would require two reels), technically you could just buy one 7/8 reel with a spare spool. That way, the one reel can be used for either rod since you'll have the appropriate matching line on each spool. Of course, the reel would only be "upsized" when used on the 6-weight.
I recently tested the awesome new Ross Gunnison reel in a 7/8 size on a 6-weight G. Loomis IMX PRO. Here's a comparison of a few specs between the 5/6 and 7/8 Ross Gunnison:
5/6 Diameter: 3.83 inches
7/8 Diameter: 4.03 inches
5/6 Weight: 6.70 ounces
7/8 Weight: 7.36 ounces
5/6 Backing: WF5F 115 yards
WF6F 95 yards
7/8 Backing: WF7F 175 yards
WF8F 150 yards
Price: $395.00 for either reel
As you can see above, the larger reel weighs less than an ounce more, costs the same, and gives you both more diameter and far more backing capacity.
Other benefits (though in my opinion not as noteworthy) can include slightly less line memory due to the larger spool diameter, and possibly more available drag power with the larger reel.
Matching the rod and reel designations exactly will always be the most common way to put together a setup, but upsizing can be a viable option or even a better option depending on the application. Keep it in mind!