Because specific mortise-and-tenon sizes vary by application, there's no universal rule of thumb for sizing this type of joint. But as a general guideline, when rails and stiles are the same thickness, make the tenon about 1⁄3 the total thickness of the tenoned workpiece, and about 1⁄2–2⁄3 as long as the width of the mortised workpiece. Though it's not structurally necessary, cutting a shoulder adjacent to the rail's edge, as well as the face, allows you to completely hide the mortise. To save setup time, size the edge shoulder the same as the face shoulder.For example, for a matching 3⁄4×2" stile and rail, first make the stile's mortise 1  1⁄2" long, 1⁄4" wide, and 1–1  3⁄8" deep; then make a tenon sized for a snug fit without forcing, and about 1⁄8" short of the mortise depth to allow for glue.—from the WOOD shop