Typical sizes are 25mm, 17mm, 10mm, and so on. To determine the magnification of an eyepiece you simply divide the focal length of the telescope by that of the eyepiece. For example, if the focal length of your telescope were 1000mm, then a 25mm eyepiece would give you a magnification of 40 times. The smaller the focal length rating of the eyepiece, the higher the magnification. Again, it is recommended that beginners start out with a few low-power eyepieces at first.
Many people are disappointed at how bad the image actually looks with at high magnifications.
Low-power eyepieces also make objects easier to find, due to their wider field of view. Work your way up to higher power after you have a little experience under your belt. A low-cost device useful for gaining additional magnification is the barlow lens. This is a long tube with a lens in it that extends the focal length of the eyepiece and increases the magnification by a factor of 2 or 3. For example, a 2x barlow lens used with our 25mm eyepiece from the example above will increase our magnification from 40x to 80x.