Size does matter or said another way; surface area does matter.  A pleated filter of the same diameter and length will have more surface area than a foam filter having the same dimensions.  I always try to fit as large of a filter that space available will accommodate.  Excessively large K&N filters will allow, "over oiling" to improve filtering action without out hurting airflow to the engine.  Excessively large filters will reduce the pressure difference from the inside to the outside of the filter, reducing the small particles being sucked through the pleated gauze.  Excessively large air filters will increase the run time between cleanings.

K & N filters have larger holes in their gauze fabric that catches the dirt than paper filters.  This is one of the reasons that they flow better than a pleated paper filter of the same size.

Foam filters collect dirt on the inside walls of the little oil-coated tunnels where air goes through.  Dirt will pass through a foam filter once all of the little tunnels have dirt stuck to their walls and there are not any sticky surfaces inside the filter. 

There are positives and negatives to every type of air filter. 

I do not like the K&N imitations that use the synthetic gauze instead of the cotton gauze that K&N uses.  Filter oil will not soak into the synthetic gauze.  Ina short time, filter oil drains to the bottom of the filter leaving the top of the filter dry on filters that use the synthetic gauze.

The filtering action of the K&N filter is drastically reduced through improper cleaning.  I would estimate that this happens the first time the filter is serviced for about 75% of the owners of K&N filters.