How do you determine nib size?

[BigWill]

I've seen replacement nibs in #4, #5 and #6. What dimensions determine what size nib you eithe bought or need? Are there #7 #8 and #3 nibs?

I need to replace the nib in my Acme FP. It's like writing with a mop.

 

[nemo]

Newer numbering system is the approximate diameter of the feed used in millimeters.