Crank length doesn't influence your gear inches, but they do influence your overall machanical advantage. Longer cranks give you more power, but force you to move your feet in larger circles, thus limiting your cadence. Shorter cranks give you less power, but you can sping faster. However, the tiny differences in crank lengths are so small, you are unlikely to notice a difference and certainly aren't going to make much of a difference to your overall power.