A SCI0 cel must be less than 2,116 pixels (46 x 46) or it will not respond to (self setMotion: MoveTo xPos yPos).
If you try to get around that with something like this:
(if (> (self y?) 75)
(-- yPos))
(self posn: 160 yPos)
)
You get major graphical glitches with cels containing over 2,116 pixels.
This behavior is exactly the same in DOSBox and ScummVM. It's a bit annoying.