Stand-over height used to be a good method of frame sizing for a traditional diamond bike frame. About zero to one inches of clearance between the top tube and crotch (bare foot) was a decent way to determine if the bike was the correct size.
With today's non-traditional frame geometries and frame types, stand-over height goes out the window. There is no frame sizing that is considered "standard," and each manufacturer may measure their frame size in a different way and from a different point. Some brands measure from the center of the bottom bracket to the top or center of the top tube, while others measure to the top of the seat tube.
Compact frames may use a "virtual top tube," and even this point of measurement may vary from manufacturer to manufacturer. Some frames now come in just three sizes -- small, medium and large. This sizing range is great for manufacturers -- as they only have to produce three frame sizes. However, it leaves gaps in sizing and makes it difficult to get a good fit.
The net of all this is that the 51 cm bike you are riding now may be different from a 51 cm bike in another brand; therefore, it is important to define the sizing method used. Manufacturers generally post sizing guidelines on their websites, or at the very least geometry specifications. A good bike shop that carries multiple brands will refrain from simply "eyeballing" measurement, and rather look up the correct frame size for your inseam.
Because sizing is based on inseam measurement, it is important to know that measurement. This can be done by standing bare foot with your back against a wall and placing the spine of a one-inch thick book against your inner leg and snug against your crotch. Make sure the book is flat against the wall forming a 90 degree angle between the wall and top of the book. Measure from the top of the book to the floor. If you used inches, convert the inches to centimeters by multiplying the measurement by 2.54. For a road bike with a conventional frame, the frame size will equal approximately 2/3 of your inseam. Now take your inseam in centimeters and multiply it by 0.67 to determine your frame size in a traditional road frame.
Women generally have shorter torsos and may need a relatively shorter top tube. This is where "woman-specific" designs come in. These bicycles may be appropriate for certain men with shorter upper bodies or riders who prefer a more upright position.
Top tube length can be adjusted to an effective length using different size stems, but this can also affect handling. It is important to be in the correct frame for your body type and the type of riding you will be doing.
Manufacturers size their frames to accommodate the normal proportions of reach to leg length. If you are outside of these proportions, finding the correct frame or bicycle to fit you will be more difficult. Take your height and divide it by your inseam. If the value of this is more than 2.2, you will need a bicycle with a longer reach or top tube. If the value is less than 2.0, you have longer legs and will need a shorter reach or top tube. Generally speaking, bicycles with a steeper seat tube angle will have a longer effective top tube length.
If you are buying a new bike, I recommend checking the frame sizing on the manufacturers' websites before purchasing one. If you are on the cusp between two sizes, adjust them both and then give them a test ride to determine which one feels better. A reputable shop will not sell you the wrong bicycle size; however, I have run across athletes who spent a lot of money on a bicycle that was the wrong size.
If you are purchasing a used bike, go to the manufacturer's website and look up your model. If it is no longer listed, you can send the manufacturer an email or take it to a coach or an experienced bike fitter to determine if it is right for you. The great deal you got on a used bike will not be such a bargain if it is uncomfortable and you cannot produce power.
Matt Russ has coached and trained elite athletes from around the country and internationally for over ten years. He currently holds expert licenses from USA Triathlon, USA Cycling, and is a licensed USA Track and Field Coach. Matt is head coach and owner of The Sport Factory, and works with athletes of all levels full time. He is a freelance author and his articles are regularly featured in a variety of magazines and websites. Visit www.thesportfactory.com for more information or email him at firstname.lastname@example.org