We suggest you purchase the largest diameter you can afford. Eyepieces - Most telescopes are designed to have interchangeable eyepieces so the magnification and filters can be changed. The size refers to the diameter of the eyepiece barrel. Larger barrels have larger optics and are easier to look through. Simple tripods must be adjusted in two axes to keep on a target and they do not accommodate heavier telescopes very well.
Equatorial mounts can accept inexpensive motor drives that allow automatic tracking.
Our advice is not to skimp on the mount. The moons of Jupiter are also quite visible. Moon observations are also quite impressive, however, if moon observations are your primary objective we might suggest a good pair of binoculars which are more versatile. The best observations are made during the new moon when the sky is darkest. If possible, find an observation site in the country far away from city lights.