HI, I love Martha Stewarts white sheet cake recipe but it is too short- even layered with another layer of cake it's too short. I am looking for either a tried and true 12x18 size recipe sheet cake for wedding or wondering how I can maker Martha Stewarts cake rise slightly more- can I maybe double it?

The simple answer, is that you're not putting enough batter in the pan. The pan should be 2/3 or slightly higher in the pan.