FSA's OS-99 CSI Road Stem achieves a lot for its svelte stature. Made with a skeleton of CNC'd aluminum and wrapped with 3K carbon, the CSI is super stiff, which tightens up your steering and handling. It's super light, too, helped by the carbon wrap, the sculpted 4-bolt faceplate and titanium hardware.
Stem length is measured along the side of the stem from the center of the steerer to the center of the handlebar clamp (see diagram). Keep in mind that as a stem provides more rise, its effective length decreases. In the example above, if the +17-degree stem measures 135 mm, it actually extends 108 mm parallel to the ground.