There is no state length limit per se; the 'deadly weapon' statute, as mentioned above, only applies when used with intent to harm another illegally.
In theory, you can open carry a Bowie knife, unless local ordinances preclude it, and that's the really tricky part; Chicago has a 2 1/2" limit, although I've never known someone NOT being charged with another crime to be charged for that. Many municipalities have ordinances on length, but generally they're applied the same way-only an 'add-on' charge if you're already doing something stupid.
For what it's worth, I've carried large (4" blade) folders with pocket clips throughout the state for 30+ years, and never (even in Chicago) been questioned. Of course, I don't carry them into metal detection points and such.
Larry