It depends on the city and state you are living in. Check the related link. Keep in mind the related link refers to concealed laws, not plain sight carrying.
3 inches and under unless it is not concealed. During Hunting season the size goes up to six inches. But as long as the knife isn't concealed it could be a Katana (NOT ADVISED).
There are no limitations. Under 21 are prohibited from carrying a "deadly weapon" concealed, but only a gun is specifically defined as a "deadly weapon". Any other item would likely be considered on...