i have read that to keep gauge consistent between plain parts of a piece and fair isle sections, you can go up a size in needles for the fair isle.
When they say “up a size” what system are we talking about? If I have 2mm needles for my main part ( it’s for socks) do I go up to 2.25mm, 3mm or what?
1/2 a mm should do it (so, 2.5 up to 3 for example), but I would strongly suggest doing a quick guage swatch to make sure! I know a lot of people hate doing them, but it would be such a shame to knit up a good bit of the actual project and then find out that you needed a different needle size!