What length knife is legal for self defense?

This is an old topic but i want to be perfectly clear on what's legal and what's not. I've heard in New York a legal length blade is 2", but i've also seen that as long as you're 16 you can have any knife, that it has to be under 4", and many other differing arguments and parameters. My specific question related to a 3 1/2" spring assist knife, so not a switchblade which i realize are illegal, and another 3" peg-open knife. Are either of these legal to carry for self-defense, or is the 2" law true? Exclude building scenarios such a schools.