I usually take my calipers and make the measurement. If you are looking for just the general dimensions, the caliper is the quickest to measure the orifice. Plus, no stock heads are exactly the same. They are all close to the general casting, usually within a +/- .010" tolerance from the OEM specs.
Are you trying to match up headers?
Or are wanting the cubic area of the port?
Cubic area is quick and simple. Your valve must be closed first, pour some water into the exhaust port until it is full, next draw the water out with out spilling any of it and place it into a measured cup, or the use of a large syringe with CC measurements. Once all the fluid is drawn out of the port, then you should have an approximation of the port capacity. Some machine shops use fine granular sand to do the measurement. This is the best way to actually get a real close port capacity reading.