Couple of generic notes about The Ratio:
1. Underlying wide adherence to whatever the current Ratio is, lies the widespread, implicit belief that a) the Rohloff guarantee hangs on observance, and b) that Rohloff goodwill (fixing your gearbox without charge if anything goes wrong even beyond the warranty period as long as you aren't at fault) depends on it, in both instances as much as observing the oil service intervals.
2. The Ratio has been Relaxed twice in the last few years, that is, a greater amount of torque has been permitted, and The Ratio has become a lower number. Many, probably most of us, believe the Rohloff box is immensely strong, certainly in no danger of breakages under even stringent use at the present Ratio. The consideration of keeping faith with Herr Rohloff isn't that we aren't scofflaws but that we'd rather he kept faith with us, as above.
3. Anyone's desirable Ratio is limited at the bottom end by how slowly he can keep his balance (as Mickeg says above) and at the top end by his short-spurt cadence for instance for blending with faster traffic, pointed out in this thread already by Martin.
4. I don't know what the present practice is, but for many years Rohloff gearboxes came from the factory with a 16T sprocket fitted. By far the most common Rohloff transmission arrangement from Continental bike makers was 38T x 16T. Thorn danced to a different tune, perhaps because its designer, Andy Blance, and his wife toured the toughest roads. I seem to remember from 12 or 13 years ago that he made such a good case for a 17T sprocket that he probably convinced quite a few cyclists who didn't in the end buy a Thorn to start with a 17T sprocket. Thorn also had their own 19T Rohloff sprocket made.
4. The problem with the Rohloff sprocket is that it lasts forever. It's actually the cheapest part of a Rohloff transmission (excluding, obviously, the box itself), but who wants to throw away such excellent engineering half-worn? So you need to choose your sprocket right in the first instance, but you needn't weep if you decide later to change it before it is worn out.
5. There is considerable enthusiasm for a chain cover called a Hebie Chainglider, inside which I've proven that you can run a medium-range KMC chain for its whole service life on its factory lube without harm to any other component. This instantly reduces routine service on your bike to once a year. That's an immense boon to older cyclists growing a bit too stiff to bend over the bike often or long to clean the chain. But, while the Chainglider has a custom Rohloff rear end which fits all the available Rohloff sprockets, its chainring covers, plural, one per chainring tooth count, fit only a selection of tooth counts, and Frank Revelo demonstrated on this forum some years ago that if you choose the wrong tooth count for the Chainglider cover you can get, the result is ineffective and ugly. If you intend fitting a Chainglider -- and I recommend it very highly -- you must choose a compatible tooth count at the front. The chainring inside a Chainglider needs to be thinner than most aluminum rings, so that steel or stainless steel is often chosen, again reducing service because steel and stainless chainrings last forever, just like the sprocket.
6. There's an odd Rohloff Effect, contrary to perfectly reasonable expectations, which a few of us have commented on. You get a Rolloff-equipped bike and you ride it happily on Easy Ratio, perhaps for years. And then you notice that you're stronger, even if older, and can bring your end of a more demanding transmission. After two heart operations, instead of going down easy street as expected, when I was forced to go from a 38T chainring to 42 or 44 or even 46 because a central electric motor I wanted to try needed a dished chainring, I thought that would put me off cycling. Not a bit of it. I took to it like a duck to water.
7. In consideration of all this, I'd advise you to leave your options of different ratios and a service-reducing chain cover open by settling on a chainring of a particular tooth count, and buying it in stainless steel (I was happy with Surly's version but there were complaints on this forum of out-of-round, etc) or even cheap steel (I liked an Indian-made Amar, which after some thousands of kilometers inside a Chainglider had not even marked the paint it came in), and then taking Dan's suggestion of swapping Rohloff sprocket tooth counts.
This is the cheapest trial and error method of arriving at an arrangement that suits you perfectly which also leaves all your other options open. You can always later sell the Rohloff sprockets you don't use on, though my bet is you won't have more than one extra, which you will want to keep, as I did.
8. If you want to tell me the rolling diameter of the tyre you have in mind, obtainable from the maker, and your normal cadence in RPM, I'll make you a decision table like those at
http://coolmainpress.com/BICYCLINGHebieChainglider.html with only components directly relevant to you to reduce the confusion.
