The answer depends on many factors. The most important one: What is the vertical tail needed for? This will be a combination of several functions, and the most pressing need will determine its size and location:
Counteract asymmetric thrust in multi-engined airplanes. With all engines running, the B-52 is still flyable with almost all of its vertical gone.
The reference point is not the leading edge but the center of gravity (CG), which is a little further back.
The ability of a fin to generate a yawing moment is proportional to both its area $A$ and its distance $l$ behind the CG. It is therefore also proportional to their product. This has units of $L^3$ so it is known as the tail volume $V$:
$V = A.l$