painted-girl-on-bike-mWomen 5'0" and shorter have historically had a very difficult time finding bikes that fit.  For those who can afford a custom bike, it's not a problem, but since custom bikes can cost in excess of $5,000, that's not an option for many women.  There are now more options than in the past:  both road and mountain bikes are available in very small frame sizes (42cm as the smallest frame in road bikes and 13" as the smallest in mountain bikes).  A few manufacturers have bikes in XXS sizes (for example, Yeti Cycles has a full-suspension cross-country bike in XXS).

Some mountain bikes have a steeply-sloping top tube which makes straddling the top tube easier if the rider stands close to the saddle (just in front of it), but there can still be a standover height problem closer to the front of the bike - and that is where most riders end up, in an emergency dismount!  An alternative top tube style that works much better for short riders is a top tube with a V-shape, where the center of the top tube is lower than the front and the back.  This allows the rider to stand comfortably over the center of the top tube.  This makes emergency dismounts much safer!

Road bikes do not have the V-shaped top tube option.  Small frames tend to use sloping top tubes, but even that is sometimes not enough for comfortable and safe standover height.  A very small frame with 650c wheels effectively lowers the top tube 1-1/2" and allows very small bikes to use normal frame geometry that wasn't awkwardly "tweaked" to accommodate standard sized wheels.

If a rider cannot find a very small bike that is within her budget, one option (other than going custom) is to get a child's road bike.  Effectively a 40 or 42cm bike with 650c wheels, kids' road bikes are miniature adult road bikes, and most are less expensive.  The compromise is in component quality, but components are something that can be upgraded according to the rider's preference and budget.  Larger kids' mountain bikes also work well for very small women.  They have 24" wheels as opposed to the standard 26" wheels, effectively lowering the standover height by 1".  Again, there is a compromise in components and frame weight, but for the budget-conscious rider, it's a great option.

Performance-oriented short riders will probably need to explore the custom bike option if they cannot find an "off the rack" bike that is a great fit.