18" barrel length, 26" overall length - measured as a straight line along the barrel, NOT on a diagonal from muzzle to pistol grip.
Be aware - cut your barrel to at least 18.5 AFTER you have measured it a bout 60 bazillion times to make sure of the length. Realize also, most barrels are not straight tubes on the outside, but tapered. Without the proper jig, you can wind up cutting the barrel improperly, resulting in it throwing any semblance off a pattern of the mark. You will also have no choke at all, and the barrel ends will need finishing and a cold bluing.
Have fun, but stay safe