Traybake Cakes

Hi Lindsey,
Using Delia's all in one method for a sponge cake, I would like to bake a vanilla traybake in a 15 x 11 x 2" tin. What quantities should I use and how many pieces of cake will it provide? I experimented with a madiera recipe but it rose in the middle and cracked which would have made icing and decorating it difficult.
Thanks.
Pat.