In the game we just played, for my own personal interest, I tried to keep to
G55's idea of no more than the box size for posts. I can't say it was a success.
However, I do think it would be a really interesting way to play.
BG is right that the box sizes are really different on phone vs PC but perhaps a rule of something like if on PC then the box size and if on phone then two times the box size.