Docking ports can only be docked to docking ports of the same size.

Don't cycle through docking ports which are a different size to the port on the current ship. This might be difficult if the current ship has multiple docking ports of different sizes.

"if the current ship has multiple docking ports of different sizes"

• well, firstly, it would work if there's only a single docking port on the vessel, which should be most vessels docking to something else.
• secondly, if there are multiple docking ports, we could use the "control from here" port (assuming we can detect that programmatically and the player has set that).