A regular soup plate is wide and shallow and has a diameter of between 9 and 10 inches. A soup-cereal bowl is between 5 3/4 and 8 3/4 inches in diameter and is a bit narrower and deeper than a soup plate.
The soup plate also has a flanged, 1- to 2-inch wide rim. It is up to 1 1/2 inches deep and has a well that's 6 to 7 inches wide. It is one of several types of soup bowls but is the only one used for formal dining.
The soup cereal bowl may or may not have a rim and is for casual dining. It's also called the oatmeal bowl.