You need a big bowl if you want enough calories to make it worth the effort. 1/2 cup oats is only about 40g, so you'd need 13 servings to meet the standard 2000kcal daily requirement (which is too low for anybody who exercises regularly).

I think 100g is a more realistic serving size, and that will spill and make a big mess if you try cooking it in a normal breakfast cereal bowl. I have a bowl about twice that size and I still cover the bowl with plastic wrap to avoid spilling any.