The diameter is approx 18” and comfortably fits a 15-1/2” full kiln shelf. It’s 24” tall on the side, but keep in mind that the bottom shelf will be supported by a 4.5” x 9” x 2.5” soft firebrick.
To get a better look at the kiln size and standard setup, you may wish to watch this video of Cindy Bracker setting up and firing our raku kiln. It’s on YouTube here: https://youtu.be/2nWH6c3aZ8o