You can calibrate your system to have real world coordinate correspond with pixels. There are three type of correction to obtain the real world measurements from your image: simple calibration (so many units equal so many pixels in the x and y direction), there is perspective calibration(if your camera is at an angle), and non-linear lens distortion. These are all described in more detail in the Vision Concepts manual
LV6\Vision\322916a.pdf
There are also shipping examples to help illustrate these methods under labview6/examples/vision/2. functions/calibration/
Hope this helps,
Brad