Here's the conversion from market grade mesh sizes(openings per inch) to microns:
From what I can tell, most tea balls from brewing sites and fine kitchen strainers are #40 mesh (389 microns).
The higher the mesh size, the more fine the mesh.
The higher the microns, the more coarse the mesh.
It can be confusing.
This link shows many meshes with size comparison. They list the #40 mesh as "coarse" and #50 as a "medium."
#50 = 282 microns
#40 = 389 microns
I think i may buy the tea ball size #40 for pellet hops. It may not stop 100% of the hops, but it will be close and allow better flow. Any thoughts?