According to One Forum and local dealers, excepting the Boston Ordinance anecdottally noted as not enforced, I have been unable to find any specific legal limitation on knife length.
However, all the specific types of 'weapons" quoted verbatim in another post/thread outlawed in Mass. (dirks and daggers and stillettos) are illegal as noted.
The length limitation ordinance in Boston, allegedly, was enacted after 9/11.
I think when a concerned citizen is diligently searching for law on a subject, and can't find anyhing on point, it creates and has created a very dangerous situation.
I have not delved into "concealed" weapons.
I have a matrix for the carrying and transportation of regulated weapons which I will try and post ASAP.