Why is a "table size" in the Settings menu useful? Doesn't calibration handle everything?

Calibration's purpose is to ensure the 2:1 length/width ratio is dialed in. However, it does not help with setting the visible diameter of balls. Providing the table size allows the program to calculate the proper ball diameter as a proportion of the table size, resulting in a correct size ball image (often 2.25", though other sizes are supported) on your table.