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This cardboard won´t work as there is big difference in weight.Here’s a suggestion. Get a big cardboard, cut one to the specific size of 11 inch iPad, and one to 12.9 inch iPad, see if that will help you feel what is acceptable to you.
I didn’t have any problems deciding what one to get, as soon the 12.9 came out with The 256gb hard drive, I immediately went and bought it. Best decision ever. then I upgraded to 2021 M1 mini led 12.9 inch, best decision ever. It’s smaller and lighter than my old iPad Pro. The smaller ipads screen size is too small for my personal enjoyment.
Some prefer small screen, some prefer bigger screen, and some prefer movie theatre screen size. Every individual person is unique in their preference. Chose what fits YOUR style/use/preference.