For example, if the mirror was a 10-inch you would use 10 x 10 to give you 100. Do the same thing with a larger telescope. The answer you get is the number of times the larger telescope is to the smaller one. It's because the power of the telescope is the total area of the mirror's surface and not the diameter of the mirror. Think of the mirrors as two rain buckets.
Telescopes work in the same way except they collect light.
You browser does not support Java Applets, Upgrade. There are 4 options in this applet to get what you want. On the top you enter your telescope size in inches or centimeters. Then enter the F-Ratio of your telescope. Just enter numbers for telescope size and f-ratio.