Ballet Rosa's sizing is French inspired, in general the sizing corresponds as follows: 
T36 = Extra Small
T38 = Small
T40 = Medium
T42 = Large
T44 = Extra-Large
We would however recommend that you size up if you are in between sizes; especially if you are ordering a high neck style such as Mirielle, Darra, Amelie or Avery as they do tend to be on the smaller size due to the style. 
Ballet Rosa Size Chart Size Guide from The Collective DancewearBallet Rosa Size Guide Size Chart from The Collective Dancewear