I have the measurements for two of Mr. Magic's Square Circles.
Square Circle Folded (Plastic) by Mr. Magic 17x13x2.75 inches.
Square Circle (net) by Mr. Magic 14x9.3x9 inches.
If you are interested in more information on the square circle I highly recommend Tarbell 82. Dan Harlan covers several ways to make your own, to your own specifications.