I'm just wondering what the R=2N means. I guess it's something like "real bar = 2 notated bars," but does anyone know?
Yes, this nomenclature indicates that "one real bar is equivalent to two notated bars."
This is based off of the work of William Caplin, who basically views 8-measure units as the standard lengths in the Classical style. In order to account for lengths that differ from this 8-measure viewpoint, he introduces the distinction between "real" and "notated measures."
Thus a 4-measure phrase is R = 1/2N, because the real measure is only half of the notated measure (and thus the 4-measure phrase is understood as actually being an 8-measure one). This is really common in slower movements.
In contrast, a 16-measure phrase is R = 2N to match the standard 8-measure length; this is common in faster movements.