While in my local Walmart yesterday I came across some Froot Loops, Frosted Flakes, and Apple Jacks cereal. They were a two-box combo with two Transformers cut-out promos on the back of each of them.
The cut-outs were either 4"x6" or 5"x7" in size {But I'm guessing the larger 5x7}.

I posted a pic of them on the FB Promotional Trading Card site, if you want to see them.