Primed stile and rail doors with a flat panel are a popular choice. The thickness of that panel varies by the thickness of the door.
Doors that are 1-3/8″ thick will have a 3/8″ panel.
Doors that are 1-3/4″ thick will have a 5/8″ panel.
These linearts shows a 1-3/8″ thick door with a 3/8″ panel on the left and a 1-3/4″ thick door will have a 5/8″ panel on the right. The image in the middle is a 1-3/4″ thick door will have a 3/8″ panel on the right. The outer images show a proportional sticking area while the center image has a larger than normal sticking area.