I have an Orvis Zero-G 9' 5wt rod and I use a Battenkill large arbor II model. The III models are too big, in my opinion, for a 5wt rod. They're heavier and the setup isn't balanced. It's simply overkill on a 5wt. Plus the II models saves you $25. I sometimes even throw 6wt line on the II model reel on my 5wt rod. Capacity isn't an issue if you do a little less backing.
Throwing a wrench in, take a look at the Clearwater II large arbor for line weights 4-6. That is a really good quality reel for $100. In my opinion, the only time to go nuts with reels is if you're planning on fighting fish that can peel line off your reel, like salmon/steelhead or some saltwater action. You likely won't be doing that with a 5wt. I've fought some 20"+ bull and brown trout on my 5wt setup and while they can pull some line off, it isn't screaming off the reel so the Clearwater can easily handle it. Give them a quick look at least.
One negative of the Clearwater is you can't buy a spare spool for it. BUT...you could buy 2 Clearwaters for the price of one Hydros (give or take $20). If you DO decide to get a Hydros, spend the extra coin and get a spare spool. Orvis likes to discontinue these large arbor reels regularly and in 5 years you'll be kicking yourself for not grabbing an extra spool while they were readily available. I am still scouring ebay and sites for a spare spool of an Orvis reel I bought in 2008. So far no luck.
