First, it depends on what kind of bike you want i.e road or mountain.  Road bikes are measured in centimeters, while mountain bikes are measured in inches.  Also, sizes vary between brands of bikes in both categories.  For SMALL size mountain bikes, I would recommend looking at Trek and Specialized for the best fit.  Others are out there but these two seem to fit small in general and have paid the most attention to size for smaller folks.  The same can be found for road bikes but there are also some road brands that have even smaller wheels sizes (650c) for shorter people.  That will often cost more and most bike shop people can help fit you on a stock bike (700c wheel size) with a few changes in stem length / height and handlebar configuration.  Other brands to look at are Bianchi, Giant and Raleigh.  These brands will all have a good fitting 50cm or 52cm bike that can easily be made to fit.  Hope this helps.