There's a company that sells record-it-yourself video greeting cards. They also have a 'keychain' fob:
Packaged Size: (T1) 3.5 x 2 in
Screen Size: 2.5”
From their commercial products page:
Dimensions: 1.89” (W) x 1.41” (H) | 48mm (W) x 36mm (H)
Resolution: 320 x 240 px | Aspect ratio: 4:3
I bought a card from them several years ago, and IIRC, it works reasonably well. I thought this might be a source of a screen for a tricorder (with a little disassembly). It would only play a single 'program,' but with a little selective editing you could have a number of 'readings' appear on the screen.
--Paul E Musselman