Life would be wonderful if everything about an engine were "longer for torque, shorter for horsepower." Unfortunately, this just isn't the case for velocity stacks, which lean heavily on a specific engine's parameters and powerband. The science and formulas are a bit complex, but the end result is a supercharged engine that requires no supercharger.
Air has mass, it has inertia and it's compressible like a spring. If you shove air into an object -- like the back of an intake valve -- at the speed of sound, then the air isn't going to just stop dead; it'll bounce back up like a spring. When the air bounces back up, it either hits the bottom of the throttle blades, or "bounces" off of the air outside the intake runners and back into the tube. If you happen to open the intake valve just as the pressure wave bounces back toward the valve head, then the pressure wave will shove air and fuel through the valve and into the cylinder. This effect is similar in effect to supercharging, but only works within a certain rpm range.
A number of factors come into play when discussing the ideal length for a set of car or motorcycle velocity stacks. Velocity stacks are essentially tubes that bolt onto the top of a carburetor or throttle body, and serve as extensions of the intake runner when the throttle blades are open. This last is a very important consideration; when the throttle is closed or half-closed, the stacks contribute very little, since air pressure bounces off of the back of the throttle blades. The most important factors you'll need to consider are: the camshaft's advertised intake duration, the target rpm of the engine, whether the engine is two- or four-stroke and the packaging constraints you're working with.
The first two things you'll need to know are how long the intake valve stays closed (in degrees of crankshaft rotation), and how many times per second the valve will open at your target rpm. As an example, we'll assume that a given cam has an advertised intake duration of 270 degrees, and that we want to tune for 7,500 rpm. A four-stroke engine rotates 720 degrees (two full turns) per intake valve opening, so subtract 270 degrees from 720 degrees (equals 450) to determine how many degrees the intake valve stays closed. Next, you'll need to know exactly how many times per second that occurs. So, divide 60 seconds by your target rpm of 7,500 (60 seconds / 7,500 rpm = 0.008 seconds). Since the intake valve only opens on every other crankshaft revolution, multiply that by two (0.008 seconds x 2 = 0.016 seconds) and you have the frequency with which your intake valve opens.
Once you have the length of a cycle (0.016 seconds in this case), you multiply that by the number of degrees the valve stays closed (0.016 seconds x 450 degrees = 7.2). Divide that by 720 (7.2 / 720 = 0.010), and you have the time that it takes from intake valve closing to re-opening. Now you have a target; the intake wave needs to hit the valve at exactly 0.010-second intervals. Since intake waves travel right at the speed of sound (13,536 inches per second), multiply your frequency in seconds (0.010) by the speed of sound in in/s (13,536 x 0.10 = 135.36 inches) and divide by two (135.36 / 2 = 67.8) because the wave has to travel up the tube and back down. Now, you're very close. You could use a 68-inch tube, but that's obviously too long, so you can divide by 2 to catch the next bounce-back. That 34 inches is still too long, so divide by two again (17 inches), and again (8.5 inches) until you get a length that will fit in your chassis.
As you can see, there's not hard-and-fast "longer stacks are better for this" or "shorter stacks are better for that" rule. It's all about tuning the intake length for your particular application. Bear in mind that this intake length measurement goes from the back of the valve to the tip of the velocity stack. So, that "8.5-inch" intake measurement might consist of two inches of intake port in the head, two inches of intake runner, three inches of throttle body and a tiny 1.5-inch velocity stack. Two-strokes require a slightly different approach. For a two-stroke, you'll measure from the end of the intake port at the piston, you'll change every "720" you see in the above formula to "360" (two-strokes have one power stroke per revolution) and the intake duration as dictated by your head's intake port dimensions. The quick way to do this is to use an online calculator; just bear in mind that most are calibrated for four-strokes, and that you want to tune it for the third or fourth wave (bounce-back).
