unterhausen is right. this is per Rohloff's website.

To prevent overstraining the hub, a minimum sprocket ratio of 2.35 must be used. With the available sprockets these minimum ratios are achieved by: 40/17, 38/16, 36/15 and 32/13. This resembles a derailleur transmission of 22/34. Larger chainrings can be used without exceptions.

I run 42/16 and top out in the high 20s downhill.