The perfect cereal bowl should be able to hold a good amount of cereal, however it shouldn't be wider or deeper than the length of the spoon you use to eat cereal out of it. Depth is more important than width, as a wide bowl lends to more spillage as you walk around. A lightweight bowl also aids in the mobile cereal eater. Having landmarks on the bowl is also helpful, as it will allow you to get consistency with repeated use, in this instance landmarks are lines or ridges inside or outside of the bowl.
