First, let's just drop the term 'long rod.' You can get 2handers as short as 10 feet up to (that I know of) 20 foot. From there you choose an appropriate rod length and line system for what you want to accomplish.
Is a rod over 14' in length appropriate for smaller streams (up to 100'ish foot wide) or new casters, probably not unless the target fish get damned big. More is not better in many instances. Using the Rogue River as an example, at the top end a 14' rod is more than enough to cover 95% of your fishing. Drop down to the mouth area (Gold Beach) there you've got one hell of a lot more river in front of you. There the ability to 'Reach out and touch somebody' really comes into play.
Next question to toss around (and there is no universal school thought on this one) is what weight rod do you need for your fishing? For Steelhead (there are some notable exceptions that come to mind here!) A six or a seven will cover you. Higher water/bigger fish (10'ish pound to even 20) a seven should be considered. Fish below 10 pounds a '6' will cover your bet(s). Hell, I've landed my share of fish in the 10# range on 5wts.
And yes, a '6' sounds light but you can really put the horse power to a fish with a 6. (Remember, a 'spey 6' 2 hander (probably) equates to at least a '8' in a single hander.) For a simple example of this just compare the casting grain weights of a single hander to a 2hander. Worlds apart! Even with the same 'line number' on the blank.
Anyway, back to the point (hopefully
) I'd look at rods from 11' to a max of 13 for the conditions outlined above. Line weight? Flip a coin but a 6 or a 7? Last thought is if you've already got shoulder problems, go 2hander. If you find you're still having 'problems' it's odds-on you're NOT letting that long lever do its job under normal fishing conditions.
"Your miliage will vary.''
And just my .02 cents.