Quote from Robin Walker himself, in an interview with CS-Nation on why there are no females in the game.
"Unfortunately, we don't have a good reason for this. When we started we wanted to ship male and female versions of each class, since that's obviously the best solution. As production went on, it became clear that we couldn't afford the memory that would be needed to do that, and still maintain the quality we wanted. Looking back, we should have shipped some of the classes as female and some as male. I think the only reason that didn't happen was due to the team hoping that we would still figure out how to do it the "right" way."
They COULD turn this around and say the Pyro is a female, but it's unlikely.
He's not the shortest though. Still above 5'8" at least. The Engineer being the shortest. He's also voiced by a man and if the Pyro WAS a woman, she would be very ugly from the posture and very small head.