Fitting a bike to a rider is an important process that directly affects the ride quality and comfort of a bike - but it's one we can't tackle online.
The best fail-safe method for determining proper size is to stop by your local Specialized dealer and let them provide you with a technician's fitting. The experts at your Specialized dealer are skilled at bike fitting and will help you find the appropriate size to fit your individual needs. You can also take the opportunity to test ride bikes in different sizes to see what feels best for you. We recommend the following when test-riding for size:
Pay heed to how your body feels differently on the different bike sizes. Is your torso cramped? Are you too far extended? Are you positioned too uprightly? Too aggressively? Also pay heed to how well you can control the bike in different sizes. Does the bike feel too snappy and uncontrollable? If so, it is probably too small. Is it too sluggish and delayed? If so, it is probably too big.
You can find contact information to set up a test-ride at your nearest Specialized dealer here: http://www.specialized.com/us/en/dealer-locator
There are many factors that go into determining which frame or bike size is best for you. Fitting a bike is similar to fitting clothing in that even though one particular size might be the size which is intended for your basic measurements, you may find that a smaller or larger size is more comfortable for you personally. This is because each person's physical dimensions and attributes (arm, torso, or femur length; flexibility, skill level, etc.) are slightly different, which can lead to different frame size requirements even for people of the same height.
If you are between sizes, we generally recommend going with the smaller size, as is it easier to make a smaller frame fit a little larger than the inverse.
Differences in bike sizes are felt most in the relative distances between the pedals, saddle, and handlebars. All of those distances can be manipulated to a degree on any frame size by swapping out crank arms, seatposts, or stems. A bike or frame of optimal size will generally have you riding comfortably with stem, seatpost, and crankarms all at middle lengths and not at either extreme.
It is possible to provide rough guidelines based on your height and inseam length, but these are only a starting point for a proper fitting with your local Specialized dealer. eBicycles.com has a great resource which provides this rough guideline here: http://www.ebicycles.com/bicycle-tools/frame-sizer
If you have additional questions in regards to sizing, feel free to contact your local Specialized dealer or email us at firstname.lastname@example.org. You can also call us M-F 8:00-18:00 MST at 1.877.808.8154.