Bike Frame Size Advice

Executive Summary about Revisionist Theory of Bicycle Sizing by Sheldon Brown
As a guide you need to get the basic frame size correct and then you can go about adjusting reach, saddle height and postion by using different sized bike components.
You would stand over the frame of a bike, and if there was an inch or two between the top of the top tube and your tender parts, that was the right size. Bikes commonly came in frame sizes two inches apart, so there was not much question whether the 21" or the 23" was the "right" size.
In the world of mass-produced bikes, the difference between different size bicycles was that the larger sizes had longer seat tubes and head tubes , so the top tube was higher. This was usually the only difference between frame sizes.
In a given model, the height of the top tube would vary, but the length of the top tube and every other part of the frame would be same, whether the bike was a 19" or a 25".
Modern bikes are generally built with proportional sized frames, the smaller sizes have shorter top tubes, and the larger sizes have longer top tubes.

Generally, when you see a single number listed as a frame's "size" that number refers to the length of the seat tube .

Even leaving the inches question out of things, there is the question of where the seat tube ends:
The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube.
Some other bikes that have seat tubes that protrude farther than normal above the top tube measure as if they were measuring to the to the top of a seat tube with normal protrusion.
Some bikes are measured to the top edge of the top tube, even though the seat tube protrudes higher up.
Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.
Bottom line: seat tube "frame size" numbers are nearly meaningless unless you know how they are measured!
Seat tube height is no longer the most important frame dimension. More determinant of the actual way the rider will sit on the bike is the top tube length.
Now 250 mm and 300 mm seatposts are stock items, and a variety of excellent handlebar stems are available. There are a number of sizing systems available today, which require various measurements of the cyclists body and reccommend frame sizes on this basis. The FitKit makes reccommendations for a particular seat tube length, and a range of top tube lengths with corresponding handlebar stem extensions. For instance, for a particular rider, it might suggest a 58 cm seat tube with a combined top tube and stem extension of 66 cm. The "by-the-book" fitting method would then be to select a suitable bike with a 58 cm seat tube, measure the top tube, and install the reccommended stem.
Comparing two frames mainly in terms of top-tube length is only valid if both have similar seat tube angles. Generally, each degree of difference corresponds to about a centimeter of top tube length. Thus, frame "A" with a 58 cm top tube and a 72 ° seat angle can give the same riding position as frame "B" with a 57 cm top tube and a 73 ° seat angle.

On a bike, the weight of your body is supported at 3 locations:
The saddle supports your butt.
The pedals support your feet.
The handlebars support your hands.
Shoulder pain often result from inappropriate handlebar adjustment. The usually recommend position for a racer is the "KOPS" position, which usually works out well with seat tube angles in the 73 °-75 ° range.
Low handlebars provide a leaning-forward riding position. The sprinter's upper body is perfectly comfortable with a very low relative handlebar position.
Thus, the ideal handlebar height with relation to the saddle height is a function of the intensity with which the cyclist pedals. The cyclist who pedals all the time will be confortable with lower bars than the cyclist who coasts down hills.
The "7" shaped handlebar stem gets its shape from a historical accident. With the smaller frame sizes used now, the "7" shaped stem is an atavism, a stylistic holdover from an obsolete technology. There is a trend to use "mountain-bike type" stems on road bikes, and it really makes a lot of sense.
Let's say a given model comes in 25, 23, 21 and 19 inch frame sizes. If you want to build a 19" frame with full-sized wheels _and_ a level top tube, you wind up with an itty-bitty head tube and steerer. This creates issues with the headset and handlebar stem, so they can't go below a certain minimum head-tube length.
If you raise the bottom bracket, you can make a frame you can call a 19" and still have the level top tube and a reasonable head tube. This is no longer that common, since the sloping top tube eliminates the problem, but short riders looking at older used bikes should beware this scam.