As an ex-racer and TT-er, and female, I'm agreeing with Colin here - if you can average 30kmph (19mph) for 30 minutes on a real road circuit, untrained and recently injured, you'd be going well and should get yourself along to a club for training.
FWIW, size doesn't really have a lot to do with it - I'm 5'6" and (when slim and fit) of muscular build and raced against a woman of 5'3" who was tiny, slight almost child-like and weighed about 15kg less than me. But we were for the most part evens overs mixed terrain. If it was dead flat, my greater absolute power output told and I was faster but if there were longish or even worse, steep, hills her lower weight was the deciding factor as her power/weight ratio was better than mine. So being tall means you'll almost certainly be heavier than shorter girls (in racing, no one's going to be carrying much spare weight) and your air resistance will be greater so you'll have to be proportionately more powerful to compensate. In men's racing, taller and heavier men tend to be the sprinters or (like Cancellera) time trial specialists. In women's racing, there seems to be a bit less specialisation - no idea why and someone more expert in this area may contradict me anyway.
