well, if you already have a road bike and you're using just one gear, just try that gearing.

of course different gearing combinations can be equal, i can't explain the mathamatics of it, but a "small" cog combined with a "medium" chainring is going to give a similar gearing to a "medium" cog with a "big" chainring. one advantage of a big chainring and medium cog is that it takes longer for them to wear out.

"gear inches" refers to the diameter of the drive wheel on the really old school "high-wheel bikes".