The touchpad is 104mmx75mm, but an AttrPalmSizeThreshold of 800 is too
  aggressive, and even relatively-small fingers and thumbs register as
  palms sporadically, stopping the mouse until you lift your hand and try again.

1600 was chosen because it's the point at which my fingers and thumbs,
  held at a very low angle, stop registering as palms, so it should
  acommodate bigger fingers.

I don't know if the [Apple Touchpads USB] default of 800 needs to be
  updated too, or if it's a quirk of this particular touchpad.