The Gen1 Enterprise will serve as a large spaceport for smaller spacecrafts. Three large, hinged, motorized doors will be located on the underside of the saucer haul. After a door swings open, a visiting spacecraft enters the Enterprise and then the door will close. Thus the doors provide entryways for visiting spacecrafts into the Enterprise’s saucer hull where they are then docked.
Three doors are used to provide triple redundancy for safety and reliability. Inside the doors, hangars exist where the visiting spacecrafts can be maintained, refueled, and cargo can be loaded and unloaded. And of course visitors will come aboard and leave the Enterprise via the smaller spacecrafts that are coming in or leaving through the hinged doors in the saucer hull.
Each door is approximately 250 feet by 250 feet. This will be more than large enough to accommodate a large spacecraft that may request to be docked. For reference, each door is larger than a 747 aircraft as can be seen by the diagram below. And each door is much bigger than a Space Shuttle which is also shown below.
It should be noted that while there are three doors for redundancy, they do not have to all be in active service during normal operation of the ship. Two doors, or even one door, should be adequate for handling traffic to and from the Enterprise. This allows the sections inside the Enterprise above the other doors to be normally used for purposes other than being a hangar for spacecrafts. This allows for more efficient use of the interior spaces of the ship and aviods wasting space. However, if an emergency ever arrived, and these unused doors need to be opened to accommodate spacecrafts, this can be done on relatively short notice.