What micron for Dry Ice extraction?

What bags do you go through on a dry ice extract?

120? 220? 90?

And how does it affect purity?

I'd say 160...but that's just me. Go with the 120 and you'll get a tad bit less yield but less plant matter

I've started using the 220 for Dry Ice Hash. Shake very gently or no matter what size you use you will have plant matter in your hash. I work my way across the glass and scrap it up to compare with the next scrap. It doesn't take long to begin to see it start to get darker and then green. Once you start seeing green it doesn't take long for all of the plant to start breaking down and you just have a pile of finely sifted weed.
Key here is gentle. Its fast easy and will produce a nice product if handled correctly.

agreed i use the 220 until color changes from blonde to light green