[header] How to choose the right telescope [title] The diameter of the objective lens or mirror is the most important number describing a telescope. [step] Larger diameter lenses or mirrors allow higher resolution (ability to separate closely spaced objects) and will gather more light from faint astronomical objects. They also cost more. 
A good size for a first telescope is roughly four inches (100 mm). Bigger tends to be considerably more expensive and more inconvenient.