I bought the size I really am and it's still roomy in the upper body to perform massage without feeling too restricted. However I rang customer service and a lovely lady was very helpful with her advice as to which ones are true sizes and which ones are bigger than actual sizes, if that makes sense.
It may say on their website which are true sizes and which ones need to be ordered in a smaller size.
Returns were easy and I ordered several at once and kept 3 . I have previously ordered from them around 10 years ago and those tunics are still in good condition, I just wanted a change from white.
Rosie