Michigan has very weird knife laws -- best to look them up.
No maximum length, but a 3" maximum length applies if you are going to attack someone with it !?!?!
"MCL 750.226 No. Michigan law specifies that a person, with intent to use the knife unlawfully against another, shall not go armed with a knife having a blade over 3 inches in length."
Other than that only "hunting knifes" are legal:
"MCL 750.227 No. A dagger, dirk, stiletto, or double-edged non-folding stabbing instrument of any length, or any other dangerous weapon, except a hunting knife adapted and carried as such, cannot be carried concealed on or about a person, or whether concealed or otherwise in any vehicle operated or occupied by the person, except in their own home, place of business or on other land possessed by the person."