I used to live in Virginia and some of the laws there were screwy. Sheath knives knives could only be worn as part of a re-enactment even or a sporting activity like hunting, fishing, or carrying it for safety when white water canoeing. I could never get a good answer as to maximum length though Ed's 2 3/4" sounds like a good estimate. Here in Illinois 3" is considered a pretty good estimate and wearing a sheath knife in and of itself is not illegal. Where you get caught is if you are carrying it to use in an illegal purpose but that applies to folding knives too. In some states concealed carry permits apply to knives where in others, like Virginia, they don't. You just have to check on your jurisdictions and go by that.
Doug