I called the Dining number last month when I was interested in ordering a cake for a friend's birthday when we would be having lunch at Kona Cafe. Dining told me that I would be able to get a 6" cake, with a chocolate Mickey wafer, without pre-ordering. That cake was $12.50. For $24 (I think), I could pre-order an 8" cake, where I could chose the filling flavor and the character decoration. I went with the 6" cake. I just told the server when we were seated that I needed a birthday cake. They wrote her name on it & it was cute and tasty. They said that the 6" cake serves 4-5 people. There were four of us, and we only ate 1/2 of the cake. We shared our cake with family with two little princesses.
Hope this helps! Enjoy your birthday trip & dinner!