Actually, the lower the better.
I have 1920x1200 on 24"
Erm, higher PPI is better.
I guess it depends. The higher the PPI is, the more you have to concentrate to see everything. Sure, you get more information on a smaller screen, but you easily compensate by moving closer to your monitor.
If I want my screen to be tinier whilst having the same resolution, and thus increasing the PPI, I can just move my chair back a few meters. I do not sit directly in front of my screen but there's a meter between me and the screen, so that makes up for the lower PPI. (in fact, it makes it almost a requirement)
But if I want to brag about going even lower...
My TV: PPI: 47.88928630613692. => 1920x1080 @ 46"