Here's K&N's calculator for air filter size (square inches):
http://www.knfilters.com/filter_facts.htm#SELECT
I've often wondered whether folks who worry about manifolds, ports, induction systems etc. sometimes consider a small hood scoop more important than air flow through a filter. Ditto on the power loss experienced between the engine dyno and the chassis dyno.
My car has a bit of an oversized scoop to allow enough filter area. Chassis dyno results are all I have, so no real comparison is available - probably I do have enough air filter since I did follow K&N's guidance.
Hope this helps,
Tom