gnus-signature-limit

gnus-signature-limit is a variable defined in `gnus-art.el'.
Its value is

nil

Documentation:
Provide a limit to what is considered a signature.
If it is a number, no signature may not be longer (in characters) than
that number. If it is a floating point number, no signature may be
longer (in lines) than that number. If it is a function, the function
will be called without any parameters, and if it returns nil, there is
no signature in the buffer. If it is a string, it will be used as a
regexp. If it matches, the text in question is not a signature.

This can also be a list of the above values.

You can customize this variable.