Depends somewhat on whether you're venting to the inside or outside of the shop. If inside the 1 micron is your choice, but even then, you should be wearing a mask. And you should have an air filtration unit to gradually filter all the finesd particles out
If you vent to the outside, which is my preference, then you hav a lot more choices. If you are trapping the larger particles in a 2 stage box, then venting to the outside, you can probably use a 2 to 5 micron filter. I have a washable 5 micron filter in the outside box. The finer filter would quicly clog in my simple system.
If you have a commercial unit with collection into a bag below and filters on top and its inside, 1 micron. If it is outside, perhaps enclosed in a vented area like a water heater cabinet, then a coarser filter will work. I am working up to putting a double chambered setup outside which will be square with a simple plastic bag below, a circular middle section with vanes to direct the air and particles down, and on top 3 rectangular washable filters that will will have something like attic vents to let air escape. Washable filters outside are very nice and less expensive in the long run than replaceable filters.
A round or barrel shaped filter indoors wold be 1 micron, outside protected from the weather could be 5. I did a lot of research on dust collection when I set up my shop, and it was a bit scary. The finest sawdust is the most dangerous. Once it gets in your lungs,its thee for good. I never work in my shop withoug wearing a surgical style mask at least. If I'm doing any cutting, I wear a small, battery powered ventilator I got from Rockler.
I also run an air cleanup filter and fan for a couple of hours after doing much cutting. It also recirculates the heat that drifts up to the ceiling.
Hope this helps, not gospel, but these are the standards I stick to.