I know of know law which indicates the length of the blade is the deciding factor in determining legality. From what I know and understand it is the legitimate use of the knife that matters. IE: a Chef with a chef's knife, no matter the size, is a legitimate use. As you indicate, if it is in your boot, you may run afoul of the local Popo.
I arrested a guy and charged him with armed robbery when he had stolen about 20 bucks worth of cigarettes during which he was in possession of a pen knife, the blade was no longer than 1.5 inches. Granted, the guy was not a nice person and needed jailing for a long time, but the possession of a "concealed weapon" during a theft bumped if from 3-6 months for theft to 5-10 years for robbery.
And some people think Size matters......:)