I would suggest you be familiar with the Types of telescopes and be somewhat familiar with the terms in the Glossary of terms . Both of these are elswhere in the Beginner's Corner. All these principals apply to any telescope regardless whether a refractor or reflector. This is the diameter of the primary objective usually expressed in inches. We are only approximating so that you can do this stuff in your head.
More light is gathered by a larger aperture.
This means that the focal length of my telescope is 7 times the diameter of the primary objective, or a 56 inch focal length. Here we are at the crux. The magnification is determined by dividing the focal length of the eyepiece into the focal length of the primary aperture. The focal length of the eyepiece is written somewhere on the eyepiece and is always given in millimetres. Common ones are 12mm, 13mm, 20mm, 26mm and 32mm.