There is only a 3" law on the books in the Milwaukee area. In my county the laws are very non-specific so it can be hard to decided what to carry. Most of what I know is based on case law.
Basically they just make it up as they go along. I have seen police measuring knives with their palm and using the 3" rule even though that isn't accurate, but I would just rather stay under 3" to avoid losing a $100+ knife. Plus the area I live in is full of sheeple. Pulling out a Manix to open a box might just get me tasered by one of the real housewives of southern WI LOL.