Fifty Starbases from Judges Guild has an interesting system for classifying highports. There are three types of orbital facilities, of increasing complexity and capability: orbital docks, orbital stations, and orbital ports. These are each classified as basic, standard, or extensive. A basic orbital dock exists to handle cargo transfer, while an extensive orbital port is a functional space city capable of building and servicing most starships.
The book also describes many ancillary facilities and features of both high- and downports as well as information on operations. It does not include maps or plans of highports, and the starbase descriptions do not adhere tightly to Traveller canon, if that's a concern to you. (For example, class D starports automatically have orbital docks. Stuff like that gets under some gamers' skins, but I have no problem changing canon to suit my needs, so the JG stuff is very helpful to me.)