Larger diameter lenses or mirrors allow higher resolution and will gather more light from faint astronomical objects. They also cost more. A good size for a first telescope is roughly four inches (100mm).Magnification is relatively unimportant and is often hyped in advertising claims for inexpensive telescopes. In astronomy, the purpose of a telescope is mostly to collect light from faint objects, not to magnify them. Most visual observing is done between 50 and 200 power.The better eyepieces have larger "eye relief," which represents how close you have to get your eye to the eyepiece to observer clearly. If the eye relief is too small you will have to bring your eye very close to the eyepiece, and it will be all but impossible to observe for more than a few seconds at a time.