It's not a silly question at all. You can completely parameterize the answer in terms of bus volume and golf ball size, or define a "best guess" by explicitly defining your assumptions on the size of a golf ball or a bus in the question. Finally, you should at least be able to provide a magnitude.

Or, you know, you could always just ask what size a golf ball, and bus is.

It seems to me the only assumption the question makes is that a person could make some logical assumptions about the question, ask some logical questions, or ignore all assumptions and parameterize the answer.