My lymphodema physio did my measurements and worked out my size. However if you have your measurements you can look up your size. If not sure you can contact for assistance.
My physio said everyone who has nodes removed should wear a sleeve at least when flying. I was measured pre op, pre chemo and throughout chemo. I found during chemo I got fullness under the arm and also when doing full day car travelling...its the lack of normal movement I suspect. So I now wear them for any long car trip and keep rotating my arm and massaging the armpit. I know as I get tightness in my elbow which means drainage isn't what it should be.
She did laser once finished chemo and it's been great ever since.
I'd rather wear one just to be cautious.