You are right, I meant distance between mast foot and fin.
You are also right that I am simplifying. However, my take is that I will choose sail size first according to the wind. Theoretically, fin selection would be second, depending on the water state and ride objective. The smoother the water, the faster I'll go, and the smaller fin I will use. If I had a big quiver, board selection would be last step. Enough mast-foot to fin length to cover well my sail, enough width to make sure I am not overpowered by the fin, but not too much width to loose traction all the time due to chop.
Volume itself is important only if uphauling is required. Weight + 30l in my case for easy success. Volume is also practical to describe the boards since it cover both width and length in a single number, but it is not accurate at all. My Kona One behaves roughly like a 135l board but is 220l. The critical measurements are the same however.