The sizes are inner diameter for eyelets grommets.
3.5 mm die for #1,7 Eyelet: Inner diameter=3,7 mm, outer diameter=7 mm
4.5 mm die for #2 Eyelet: Inner diameter=4,5 mm, outer diameter=8,5 mm
5.4 mm die for #3 Eyelet: Inner diameter=5 mm, outer diameter=10 mm
6 mm die for #4 Eyelet: Inner diameter=6,2 mm, outer diameter=11,8 mm
7.5 mm die for #5 Eyelet: Inner diameter=8,1 mm, outer diameter=15 mm