The Bad Boy you linked to is the 3-speed version, I would go for one of the other similar models that have a much larger gear range. Unfortunately, all but one of those has disc brakes. If you're getting it for fitness riding on the road and decent bike paths, then there is no need for disc brakes - they will just add cost and about half a kilo of weight. The only model of the Bad Boy that they make with rim brakes has big, fat, 2" wide tires, which are going to be very inefficient on the road. Also, avoid the suspension fork on the front (which some of the Bad Boy models have and some don't) - that will add about a kilo to the weight and will again be unnecessary if you're staying on the roads and good bike paths. Overall, I wouldn't recommend any of the Bad Boy models for the kind of riding that you are talking about - go for something less tough and more nimble. However, I also wouldn't recommend going straight into a lightweight racing bike like blackbird is encouraging you to do. How about a Specialized Vita?

As for size, if you're buying a new bike from a bike shop then they will help you to choose the frame size and will help you to get it adjusted so that you are comfortable.