There's no specified length limit in WI. Less than 3.5-4" is a good rule to follow in most States without a written length limit, if you're worried about getting searched. If you're getting searched, the cop probably isn't happy with you, and that's not the time for you to be discovered with a vaq. grande.
Above 4", you start taking serious chances. When statutes talk about "carry with intent", in some places intent can be inferred from the fact that you had an unusually long knife. It's Kafka-esque, but that's the way it works in some states. I only skimmed a few cases, but Wisconsin courts tend to require the State prove both that it's dangerous (what knife isn't?) and that it's a weapon. A Wisconsin jury might think that 3" or longer blades are automatically weapons, since Milwaukee and Racine both have 3" limits.
Given that, I'd stay under 3" in Wis. if I wanted to be legal.
239 Wis. 2d 595 (folding knife in a belt holster is a concealed weapon)
1996 Wisc. App. LEXIS 1200 (State v Dumas, 5" blade on a folder can make it a concealed weapon unless there's an obvious legal use like hunting/fishing)