i'm from Chile so idk about US sizes and i'm going to buy some things from internet. but i dont want baggy clothes. and neither small..

in Chile i'm size M for women. but in the chart sometimes says

adult xs
adult sm
adult md
adult Lg

and on other t.shirt it says

girl small
girl medium
girl large
adult xs
adult sm
adult md
adult lg

so... i dont know what to choose! >,<

girl medium equals to...what?
if it says.

girl medium and also says adult small

which do i choose?