Alright, so since I'm getting serious answers anywhere from 5 to 40 miles across, I'm probably safe in calling it ten miles.

Although once the suggestion that its a cosmological event got in my head, I'm having a hard time seeing something other than nebula in it. (I think, though, it's safe to say that I wouldn't describe a gas density field as "terrain.")

Thanks for the input, everyone. The lack of a scale clue was deliberate; I didn't want to influence the evaluation with anything other than the mountain itself.