Measure any two of the perpendicular pieces that make up your shower's frame. These pieces are commonly referred to as the "curb" or the "sill" of the shower. As corner showers are typically installed at 90-degree angles, you can measure two perpendicular pieces to find the length and width of the unit.
Divide each of your inch measurements by 12 to convert to feet, a smaller number that better represents the curb or sill of your shower.
Multiply the measurements from step 2 together to find the square footage of your corner shower. For example, if each piece of the sill you measured was 3 feet, your corner shower has an area of 9 square feet.