The fabric is jersey and therefor very stretchy. As a rule of thumb, you can follow the sizes below (EU sizes):
Beautiful = 36/38
Marvelous = 38/40
Lovely = 40/42
Fantastic = 42/44
If you want to take good care of your blouse, you can mashine wash it at 30° and hang it to dry. Read more about washing here.
The organic cotton stretch single jersey from BoWeevil is produced in Turkey and certified by the Control Union.