The question "What size bike should I get?" comes up quite often. About 50% of the time the recomendations given are wrong I believe. Just because someone rides a bike that is too big for them yet it seems to work okay is no reason someone else should ride a bike that is too big for them also. Over the years before I knew any better I've was sold bikes that were the wrong size for me yet I seemed to enjoy riding the bikes. Having said that riding a bike that truly fits is a far more enjoyable experience. Stem length and fore/aft saddle adjustment are there to fine tune to perfection, not to compensate for an ill fitting frame. This chart is for use as a guide only. It is based on over 25 years of experience of one of the world's largest bicycle manufacturers and it really works provided the rider is not excessively abnormally porportioned. In other words for most of the population.
If you are buying online use this chart as your guide, do not go to your LBS and expect them to assist you in getting sized for a bike you are buying off the interweb. Nothing angers them more, rightly so I believe.
The matrix is self explanatory. Find your height and look for the size directly beside your height. Personal preference comes into play also. At 5'8" I'm able to ride 51cm on the smaller end up to 54 cm at the larger end of the scale. My personal preference are smaller bikes so I don't ride anything over 53cm. The top tubes start to get too long for me on 54cm bikes. Stand over height really means very little. If you choose a bike that is the correct size from the chart you can almost always be assured you will have sufficient standover clearance. I hope this is useful, feel free to add comments or ask questions.
Extra Small
5'0" 48cm
5'1" 48cm
5'2" 48cm
5'3" 48cm
5'4" 48cm
5'5" 48(49)cm
Small
5'5" 51cm
5'6" 51cm
5'7" 52cm
5'8" 52(53)cm
Medium
5'8" 54cm
5'9" 54cm
5'10" 54(55)
Large
5'10" 56cm
5'11" 56cm
6'0" 56(57)cm
Extra Large
6'0" 58cm
6'1" 58cm
6'2" 58cm
6'3" 58(59)cm
Extra Extra Large
6'3" 60cm
6'4" 60cm
6'5" 60cm
6'6" 60(61)cm
Extra Extra Extra Large
6'6" 62cm
6'7" 62cm
6'8" 62cm
6'9" 62(63)
Edit: I tried to do a matrix but it gets reformatted to fit the page so I've done it like it displays above. I have limited computer skills.