To define your size correctly you will need measurement tape. Keep it straight and do it in front of the mirror - it would be easier control correctness ща indications.
All measurements should be taken by the most convex parts of the body.
The tape should be on the same level both on front and on back sides. Breathe easily - there's no need to hold your breath or measure while inhaling.
Measuring breast and underbreast parameter lower both of your hands down.
Measuring thighs ease them and stand straight.
You gotta make it!