You're probably best off with a 50cm frame (for a road bike) or 39cm frame (for a mountain bike). Depending on how you like the fit you could adjust around some to a slightly larger (51, 52cm) or smaller (48, 49cm) frame.
If you're buying from a bike shop, you should get the opportunity to check some bikes out and see how they feel. If you're buying online, then, since you cannot try the bike out first, I'd lean towards a slightly smaller road frame, as the downsides of a frame that is too large are much greater than a frame that is slightly too small (in my opinion).
Many manufacturers of road bikes now make women-specific design (WSD) bikes; that may also aid in a better fit for your body size.
Check out some of the links below for some of these designs.
If you're buying from a bike shop, you should get the opportunity to check some bikes out and see how they feel. If you're buying online, then, since you cannot try the bike out first, I'd lean towards a slightly smaller road frame, as the downsides of a frame that is too large are much greater than a frame that is slightly too small (in my opinion).
Many manufacturers of road bikes now make women-specific design (WSD) bikes; that may also aid in a better fit for your body size.
Check out some of the links below for some of these designs.