I'm sure you'll get great replies on this topic from folks who are more meticulous than I, but here's my take on it.
You asked two questions really, balance (weight) and size (capacity).
Capacity:
Reel capacity on the Sky/Skagit should be at least 200yds of 30# dacron or gelspun which comes 50# thinner than the dacron but is less "meaty" in the hands. It may be rare that you need it but if you do it will be one hell of a steelhead and you don't want to be short. As Rick points out, the line you plan on using defines the remaining capacity of the spool on the reel. Extended bellies need a lot more than spey heads, etc.
Balance:
For me it's balanced when the rod's top end feels neutral while holding the rod where you want it to be as the working length of line swings through the drift. This for me is low and almost parallel to the water with the tip holding tension on the line with my upper arms relaxed by my sides.
This means reel, backing and remainder of line on the rod, and the working length of line out of the guides. If you're fishing 80ft most of the time then that's a good place to start.
Is it really that important? I recall this discussion on the speypages before and some would argue it's not that important while most feel it's of moderate to critical importance. Well IMHO I'm going to fish either way but when it's right it's sure nice. Your arms get far less tired over the day.
On the downside I have had rods and reels / lines balance out so well that I tended to loosen my grip on the rod and have almost had it pulled out of my fingers by aggressive summer fish, but that's a downside I don't mind having. Pretty embarassing though.
When the reel's too light or the rod's too heavy I hold the rod differently to take the tension out of my arms. It's not all that big of a deal if you find a comfortable alternate rod holding method thru the swing. Not as nice setting the hook but the job gets done.
Then there's the weight balance while casting. A very heavy or light reel influences the way the rod feels when moving the outfit around, for instance in a double spey or snap-t. We've all felt how a rod's feel changes from the store shelf wiggle test to the moment you slap a reel in the seat, almost a sensation of shortening the rod or an increase in momentum from the bottom end. Less of an issue than the swing balance IMHO, but still important and if you are balancing you would do well to pay attention to this aspect as well.
Balance within the rod design itself is important as well. As long as the rod is light enough, differences in weight between one rod and another are not nearly as important as the way the weight is distributed over the length of the blank (e.g. top-heavy, bottom heavy, etc).
Good luck, perhaps you could post your results for the next guy.