Pen-based handhelds designed around writing seem to gravitate to larger form factors than phones.

I'm surprised nobody's mentioned the similarity in size to the Apple Newton MessagePad from the 90s:

Original MessagePad: 184 x 114 mm (7.25 x 4.5 in)

reMarkable Paper Pro Move: 195.6 x 107.8 mm (7.7 x 4.24 in)

No comparison in thickness though, with the Newton being about 3x the thickness of the reMarkable (19 mm vs 6.5 mm).