The size of your curb-wall, when measured from the outside, should always be 12″ or higher in case of a sloped grading which required a retaining wall.  This measurement includes the concrete slab and might change according to the ground slope. Hence it will be underground and will come out to approximately 4″-7″ above ground depending on the location measured.  Generally the closer it is to the ground lowest level the higher the curb wall is visible above ground.