Secret menu also includes 3x3 and 4x4, meaning the number of patties and cheese. You can get these "animal style."
Trivia: INO used to let you "#x#". Until the record was set in Vegas by a bunch of drunk fratstars... 100x100.
They've since capped it at 4x4.
Some places will still let you do a NxN higher than 4x4 if you are reasonable.
I recently got an 8x8 and they had no problem with it. And yes, I ate it all, but there is no point in getting anything larger than 4x4 since your mouth can't open wide enough to be able to eat it, so you have to use a fork.
Just get 2 4x4s if you want an 8x8