Increasing the cooking time is one thing, but getting the heat in to the middle of the cake takes longer, the larger you go. There is a risk that the outside of the cake may get a little crisp before the middle is completely cooked. Putting a cake cone (inverted aluminium cone) in the middle of the cake would draw the heat in and cut the cooking time. Instead of the middle of the cake being 5 inches from a heat source (on a 10" square cake), the furthest any part of the cake would be away from a heat source would be about half that. The cake would be more evenly baked.