There are two dimensions involved here. The stem can go up and down a certain amount ... but you can also buy a stem which is longer front to back---the measurement is taken from the center of the bolt on top to the center of the clamp area (halfway through the handlebars.) For instance, I have a very short stem on one bike (40 mm) and a more normal stem on another bike (90 mm) and both are the same height above the headset ... and stems out to 120 mm are readily available.