If you look at my photos I made a t-shirt inside a black box, look at the third line. If you want to make the cake a t-shirt, how many people do you want it to feed?
You should probably use a 9x13" and carve it to look like a t-shirt. When you cut the sides to shape the t-shirt make sure you keep the cut outs incase you have to use it to make the sleeves longer.
