If your question is about plenum volume sizing, a good rule of thumb is 50-70% of the displacement of the engine, higher % as your rpm range climbs.
If your question is how to calculate the size of your plenum, for yours it is: (area of trapezoid (side)) times depth.
Area of trapezoid is found as follows:
[(Long side + short side) / 2]* height
or (3" + 2")/2 * 2.5 = 6.25 in2
6.25 * 19 = 118.75 or ~ 119 in3, which is about right.
