Most string trimmers, blowers, hedge clippers I use 3.0mm
Saws up 60cc's I run 3.5mm
Saws 70cc+, and cut off saws I use 4.0 or 4.5mm can't remember off hand.
I have been in several Piussing matches over rope size, as some of the manufacturers has 4 and 4.5 etc on their rolls, but they don't always mean
4mm and 4.5 mm, so I have all sizes, and just "eyeball" the size when I install the rope.