As I understand Illinois law, blade length is only limited to 2 1/2" in Chicago and some other areas in the state. Otherwise, the definition of a dangerous weapon is a blade 3" or longer. I found some of the statute in St. Louis is vague, but doesn't say anything about blade length:
15.130.010 Concealed weapons.
It shall be unlawful for any person to wear under his clothes, or concealed about his person, any pistol or revolver, colt, billy, sling shot, cross knuckles or knuckles of lead, brass or other metal, bowie knife, razor, dirk knife, dirk, dagger or any knife resembling a bowie knife or any other dangerous or deadly weapon, within the city, without written permission from the mayor. Any person who shall violate this section shall be guilty of a misdemeanor. Nothing in this section shall prevent any United States, state, county or city officer, or any member of the city government, from carrying or wearing such weapons as may be necessary in the proper discharge of his duties. (1960 C. 777.010: 1948 C. Ch. 46 § 8.)