I have a station where there is a bus stop, truck stop and a harbour all combined together. Now I like to add a city airport. But I cannot combine the city airport with the existing station. When I choose a small airfield instead, I can combine this with the existing station. Is there any reason for this? Am I doing something wrong? Or may this be a bug or some game limitation?
It looks like you're trying to extend the station beyond the maximum set station spread size. The small airport still fits within this limit (which is 12x12 tiles by default) but since the city airport is much bigger it most likely extends beyond the limit.