New York State (statewide):
There is NO statewide length limit for knives in the State of New York. Many specific kinds of knives are banned regardless of length, notably "switchblades" and "gravity knives," as defied in statute. There are special local laws in some jurisdictions.
New York City:
The law in New York City is vague, but goes something like this:
-Blade Length cannot exceed 4"*In New York City max is 4"
-Blade cannot open automatically (switch blades, etc) or with assist (spring opened, gravity or inertia knives)
-Knife must be concealed. (This means no visible aspect. No pocket clip, no handle, no nothing
New Jersey :
if it's a lockback and it's longer than the width of 4 fingers I think its illegal in NJ.
I just checked most of NJ limitations only apply to minors.
New Jersey
Summary: General folders OK. Single-edged fixed blades may be.
Max length: Under 18 may not possess knife with 5" or longer blade, or 10" or longer overall. No other apparent limitation.
Specifically illegal: gravity knife, switchblade knife, dagger, dirk, stiletto, or ballistic knife "without any explainable lawful purpose" (i.e. an ill-defined exemption). Manufacturers and sellers are not exempt.
Relevant laws: 2C:39-3.e 2C:39-9.d 2C:39-9.1 (Statutes, search for "knife" or "knives")