Hi Katerpillar2,
Dust mite themselves are fairly large on a micron scale but their larvae and fecal particles are in the range of 10 microns. Pollen is also in the range of about 10 microns and dander can be as small as 6 microns and sometimes smaller. There is more information about mattress encasements in post #2 here.
To be safe I would probably look for a tightly woven cotton fabric that has a pore size of 5 - 6 microns or less and is made so that the micron pore size is maintained through washing.
I am certainly no expert in mattress ticking fabrics and mattress ticking is a completely different subject from an allergy mattress encasement. I don’t think you will find a mattress ticking fabric that has a pore size small enough to be effective against dust mites and you will likely need a separate allergy encasement to protect against allergens in addition to a mattress ticking.
There is a huge range of ticking fabrics depending on the specific properties you are looking for (woven, stretch knit, feel, weight and thickness, and many others). I think that if you aren’t already an expert in mattress ticking fabrics (and I know people who specialize in this and have been involved in this part of the industry for many years and are still on a learning curve) that the most effective way to choose a mattress ticking is to talk with mattress manufacturers that use a mattress ticking that you like and see if they will share some general information about the fabrics that they use and why so you can choose a fabric that is the best match for what you are looking for.
Phoenix