Floor of an N-bit signed by an integer is usually an N-bit signed. (Strong form, case splitting, disabled by default).
See also basic-signed-byte-p-of-floor.
Theorem:
(defthm basic-signed-byte-p-of-floor-split (implies (and (signed-byte-p n a) (integerp b)) (equal (signed-byte-p n (floor a b)) (not (and (equal a (- (expt 2 (- n 1)))) (equal b -1))))))