Re: Reel Sizes
I would not recommend going with a 3/4 reel on a 5wt rod just to reduce total weight, as it will actually increase "swing weight", throw the balance off, and thus be less pleasant to cast.
In general, the line size listed for a reel is based on what rod it will balance, as well as line capacity, assuming you're talking about a typical example of that weight rod. (7-8' 3wt, 8.5-9' 5wt) So if you have a 10' 3wt or 4wt rod, it may need a 5/6wt reel for balance, and vice-versa, a 7' 5wt may need a 3/4 reel to balance it.
To squeeze a 5wt line on a 3/4 reel, I would be more likely to cut of 10-20' of running line on the back end of the fly line in exchange for 40+ YARDS of backing. Its not likely that you'll be making 90' casts with a 5wt short enough to need a 3/4 reel, but you still could hook into a backing-needing fish, and it would be a damn shame to run out. (Those wishing to chime in with "blah blah, Lee Wulff could cast 200' with a 3' cane rod, blah blah" just don't. Really not relevant here)