The pattern for the poncho is very simple and can be adapted to any size. The one in the photo I did for my sister and would fit an adult woman. I would guess this is a size S.
You can also knit the poncho using circular needles, just adding the increases in the right places.

The translation to English is now available (just scroll down the page, it is below the portuguese version), please let me know of any mistake i made :).