I figured it out after hours of testing different combinations
This is how to get the screen size of the mesh in screen in the material editor:
And this is the result:
For those who want to try it, it works, I would only warn that there is a margin of error of 0.001 only when changing the screen percentage of the viewport. Maybe someone will be interested in posting a fix to that, but for me it wasn’t worth the time. Hope it works for you