For a 170cm splitboard would you recommend...
For a 170cm splitboard would you recommend size M or L?
G3 sizing chart reflects either will work, I'd go with Large to have some extra skin to work with when trimming.
The length is pre-cut, you do not trim length for these skins, only width!
Mediums will work fine on a 170. By sizing down, you will lose about 11cm of skin, however if your board has any tail rocker, and most do, those last few cm's don't matter anyway.