You can measure the vents yourself or work with a contractor or professional to do this. Either way the attached guide offers a simple method of measuring the vents.
Although it's tempting to eyeball the measurement nothing is worse than getting excited for your system's arrival only to realize that the sizes are incompatible!
Measuring your vents is simple; you'll need the following:
a ruler or tape measure
a ladder if your vents are up high
The most accurate way to measure your vent is to unscrew the vent cover (the register), and measure the length and width of the duct box below. Check the attached document for pictures!