Air filter size calculation:
Air Filter Selection
If maximum horsepower is the objective, the size and shape of the air filter element is paramount.
Let's first consider shape. When fitting a conventional round filter on top of the engine, such as a carburetor, central fuel injection or throttle body fuel injection, we have found a large diameter, short filter will flow more air than a small diameter, tall filter. For example, a 10-inch diameter filter 2-inches tall will flow more air than a 5-inch diameter filter that is 4-inches tall. Where space permits, the height of the filter should be between 1/5 and 1/4 of its diameter.
The shape of the filter is less important if the application calls for a remote mounted filter, which includes many late model fuel injected models. Typically these vehicles will use a flat panel filter or a conical or cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.
That brings us to size.
Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them since very little air flows through this area.
A=(CID X RPM) / 20,839
A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute at maximum power
Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.
A=(350 X 5500) / 20,839 = 92.4 square inches
Source:
K&N Air Filter Facts You Should Know
Another calculator here:
Second Strike Air Cleaner Calculator
If you're not hung up on the appearance, a K&N X-Stream top (or equivalent) is an option to add effective surface area.
Just my $0.02.