It is considered hazardous only if it is not packaged so that the point or edge will not penetrate the package. There is nothing in the postal manual about blade length.
Here is the entire section from the postal manual regarding knives:
12.6 Knives and Sharp Instruments
12.6.1 Mailability
Knives (including sharp-pointed instruments such as stilettos that lack cutting
edges) with a blade that opens automatically by hand pressure applied to a button
or other device in the handle, or by operations of inertia, gravity, or both, or with a
detachable blade propelled by a spring-operated mechanism, are mailable only
when sent to:
a. The respective government’s or organization’s designated supply or
procurement officers and employees ordering, procuring, or buying such knives
for use with the activities of the federal government; the National Guard, the Air
National Guard, or the militia of a state, territory, or the District of Columbia; or
the municipal government of the District of Columbia or of the government of
any state or territory, or of any county, city, or other political subdivision of a
state or territory.
b. Manufacturers of such knives, or bona fide dealers of such knives, in
connection with a shipment made under an order from any person designated
in 12.6.1a.
12.6.2 Addressee Identification
Before delivering a shipment (or parcel) that contains an article or articles described
in 12.6.1, a USPS employee may require that the recipient identify himself or herself
as in one of the categories in 12.6.1a.
12.6.3 Wrapping
Sharp-pointed or sharp-edged instruments such as knives, tools, ice picks, and
razor blades, that are otherwise mailable, must be wrapped to protect their points
and edges from cutting through the outer carton in which they are mailed.
12.7 Prohibited Parcel Marking
For any parcel containing a firearm or a ballistic or switchblade knife, any marking
that indicates the contents is not permitted on the outside wrapper or container.
