I was commissioned to make a bunch of different eye designs for a project, and here are a dozen of them. The biggest in this picture is 25mm across and at that size, I was so sure my machine was going to choke and jam at the requested size that I also made them at 125% and 150% to find out what the minimum size would actually end up being. They came out much better than I expected at the requested size.
Others (Black and White): fav.me/d7aicz7
I make lots of patches and digital embroidery files: ethepony.ecrater.com/
If you have an embroidery machine, you can download designs from my blog: ethepony.blogspot.com