Edited with a more precise diagram:

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Using a little Pythagorean Theorem, I came up with the attached diagram. I am considering going from a Torch 9810 to a Q10, hence the example.

Rough dimensions:

9810 - 1.88" x 2.5" (3.2" diagnal) = 4.69 sq in

9900 - 2.24" x 1.68" (2.8" diagnal) = 3.76 sq in

Q10 - 2.19" x 2.19" (3.1" diagnal) = 4.80 sq in

If my math is right, the Q10's screen width is just a hair narrower than the 9900, but is 0.5" taller...basically real estate from the row of buttons. Coming from the 9810, I'd gain about 0.3" width, and lose 0.3" in height. However, my screen real estate would actualy increase by a tad. Not bad