Well, every brand of bike is different. I have to ride 14" and I'm about 5'1-5'2. Any more than that I can't even stop properly.
Anyway, I agree with the other person about the single-speed road bike. It's not safe. I know it seems simpler and easier, but it really is not. When you are riding with cars or other bikes, you have to be able to keep pace... just like with driving a car, you have to keep up with traffic at least to some degree, correct? Same with a bike.
I've seen a lot of people riding fixies in the bike lane and end up cruising through 4-way stops because they couldn't stop in time, or I would see people riding fixies too aggressively in the road... in fact some of them just flat out try and exceed the bike's capacity altogether. If those same people got a geared bike instead of a "cool" one, they would be able to do those things without risking serious injury to themselves or other people.
If you do end up getting a fixie, please for the love of all things great do NOT get a brakeless one. There is a whole subculture of people who ride fixies who believe that stopping the wheel with their shoe is more reliable than a brake. It's not reliable, it's reckless. You only do that if your brakes aren't working. I hope those things become illegal in the very near future if it isn't already