Identify the fixtures you need to vent. This includes toilets, sinks and showers.
Access the pipes below or attached to the drain on each fixture. You may need to enter a crawlspace or basement to see these pipes.
Look over the pipes for the printing on the side. The printing contains the diameter of the pipes. Diameter is listed in terms of inches. You may need to use a flashlight to see the wording.
Write down the sizes of all the drain pipes. Divide the largest of the sizes by 2 to get half of the pipe diameter. This is the size that must be used for the vent stack. For example, if the largest pipe on the drain system is 3 inches in diameter, the vent stack should be at least 1.5 inches in diameter.