These Leigh 670900 Square tenon guides come in four sizes: 1/4", 5/16", 3/8" and 1/2".
These guides can rout square cornered tenons to match any mortice length up to 5". These also have the ability to rout lap joints and other types of joints, not just mortices and tenons.
All square tenons are routed with a 1/2" router bit.