Multiplying N-bit signed * N-bit unsigned creates an N+N bit signed.
This is a nice and general theorem but it rarely unifies automatically. See also basic-signed-byte-p-of-mixed-*. Also see lousy-signed-byte-p-of-*.
Theorem:
(defthm lousy-signed-byte-p-of-mixed-* (implies (and (signed-byte-p n a) (unsigned-byte-p n b)) (signed-byte-p (+ n n) (* a b))))