Cutting with air always requires quite a high gas pressure and much more power than with Oxygen.
Sometimes a focus adjustment can help also.
In truth, I think everything affects everything else, probably you need a wide enough kerf to get a high enough gas velocity through the cut to blow the molten steel clear of the edge of the cut.
A wider kerf requires more power, and probably a bigger spot size.
Spot size is function of lens focal length, properties of the raw beam, and position of the focus waist w/respect to the material.
Generally, you find a way to conduct cutting tests quickly and zero in on the combination of parameters needed with your machine and your material.
Usually those tests don't require an entire part, just test a few representative features on some scrap material. Then you use that info to tailor your part's cutting parameters.

Jay Maechtlen
http://www.laserpubs.com/techcomm

by the time you have an air system that can properly support a laser as an assist gas you'll have spent about the same as the cost for a N2 generation system. You have to also consider the power costs of high pressure compressed air/N2 generation.

Chris Krug http://krugtech.com/
Maximum Up-time, Minimum BS