When clothes tag says size xs/tp, what does the tp mean?

Tre petite.
It's essentially the same as extra small.

In 2081, everyone is equal not only in the eyes of the law but in every way; nobody is smarter, more athletic, better looking, etc. than anyone else. hazel is of average intelligence, which means she can't think about anything except in short bursts, and george has an intelligence way above normal.