These are custom made as the size varies considerably in width, depth and length. Some are built to go with window size or perhaps the size of containers or pots they are to hold. 

The one shown in picture is 3 feet in length x 1 foot depth and 1 foot width. Constructed with frame of angle iron base with flat steel rim and curled scrolls of 1" x 1/8" flat iron. Holes are in the back rim for easy mounting on a wall.