Foam air filters, what size?

Look on some of the filter maker sites.. I would guess something fine enough to gather bulldust, which is a wee bit fine.

Finer filter.
Unifilter.
Etc.

Alister,
my guess is that you will need to find the micron size of the foam filter, Ie the actual size of the dust that will be caught by the filter media any one who sells the foam filter should be able to give you this. 5 micron is about the normal size for filter efficiency and a grain of sand is about 50 micron, one thing to remember is the foam is the actual substrate and the pollutant does the filtering so the dirty the filter gets the less flow you will get through it and the greater its efficiency will be.
Hope this helps
Pmacks

You need to have a pretty good reason (IMHO) to move away from a paper filter. Foam filters are good because you can clean them and reuse them - But they are not as good as paper for letting the air through. Paper is also better because larger particles fall down away from the paper where has the foam tends to trap them.

Gaz

Gary's comment echoes the question that immediately cam to me on reading your post alister. Why ever would you want to use a foam filter in preference to paper. Many engines in heavy earth moving equipment now use paper filters for one simple reason - they do the job more efficiently.