"Gear inches" refers to the effective wheel diameter you would have if your pedals were connected directly to the drive wheel. That's how it was with a high-wheeler - you needed a larger wheel to go faster.

You calculate gear inches by multiplying the wheel diameter times the chainring size, then dividing by the cog size. Or as a formula:
G = d * F / R.

A 52T ring with a 14T cog would be 27 * 52 / 14 = ~100 inches. (A 700c wheel is close enough to 27" for this purpose.)

The ideal gear depends on terrain and riding style, of course. I find I ride mostly in the range of 50"-60". For the most common hills around here it is nice to have a low around 32". I almost never use anything higher than, say, 80".