Stihl, bless their little hearts list both the string length and diameter in the parts breakdowns
And for 120 anything the string is 3mm ( cause they are designed in Germany )
Be careful because the USA uses both imperial sizes and a GAUGE size which is NOT the mm equivalent
OK in my previous post, I was talking about the starting string size
If you are talking about the cutting string ( usually called line down here ) that will depend upon the actual head you have fitted.
Bump heads are not particularly sensitive but the auto feed one is.
Looks like the standard 25-2 head. You can go up to about .105 line. .095 seems to be the perfect size for me as it’s a little forgiving. I also prefer the square line over the round...just cuts better at lower rpm’s.