These are alternating shutter-based glasses, and thus your screen must be capable of rendering alternating frames at very high speeds. These particular glasses only work with a CRT monitor. There are other brands that are made to work with LCD screens, but I've heard that they ghost horribly. I have a 19" CRT and a pair of the older X3D glasses based on the same technology. These work fairly well, but there is a bit of "lag" visible, although I'm not sure if it's caused by a delay caused by the cord and converter box, or because of the response time in the LCD shutters. Keep in mind, these really only work well in 3D games and movies designed especially for this type of 3D viewing. Don't expect to pop a copy of Lord of the Rings in the DVD drive and suddenly get a 3D movie. The software really only makes the video appear to be pushed back into the screen, and using paralaxing, movement appears to be "slightly 3D", until the object moving stops, and then the image looks flat again.
These glasses probably eliminate the ghosting lag, as they are a few years newer than the X3D model, and they probably work better with Nvidia's newer drivers.
As for screen size, it doesn't really matter how large the screen it, but the edges of the monitor might throw you off a bit. Depth appears to be about a foot deep at maximum setting, and 6" out.
Since games do not render a 3D crosshair by default, you can use the Nvidia stereo driver to render a "laser sight" in those types of games (it's just a crosshair rendered in perspective of both eyes, so that there won't be two on the screen).
Some people report headaches after using such a device. I haven't had that issue, and I often use them for several hours at once. This really depends on how well your eyes adjust to depth and how susceptible you are to eye-strain.