I am male, 6'1 and 195 Ib, and large sized T-shirt fits me kinda tight and extra-large sized is either just right or a little loose, and my suit size is 44L, and I ordered the Extra-large version of the white coat for Vandy. Hope it helps.
Keep in mind that this will be worn over normal clothing and also what type of fit you will want. I would go one size above what you wear for a suit and maybe extra if you plan to wear heavy clothes underneath at times (sweaters, oxford shirts, etc). If you have ever worked in a lab a good size would be what was comfortable there for me a 52.