Of course, petites don't fit human-scale furniture or clothing, so if a petite sits on a human-scale chair she will probably sit in the right pose, but be buried in the chair or floating above it. Petites can't wear system clothing, they have to have special mesh clothes, shoes, attachments, and hair. You can adjust skins for use with modable petites, and there are instructions on how to do that here.
|Petite Yabu fairy with a various attachments|
Tinies are generally little furries, chunky and small. They have the main avatar folded up inside them, and therefore they need special animations to be able to move correctly.
If a petite sits on a tiny chair, she gets folded up in a bizarre way, floating above the chair, as seen below. If a tiny sits on a petite or human chair, his body will be scrambled, too.
|This is what happens if a petite sits on a tiny chair....|