Buying my first latex piece, how do I get the size right?

So I have decided to buy my first proper latex piece and I am stuck before I have even started, because the sizing looks completely different from normal clothes.

Do I go by my usual size, or measure? And is off the peg okay for a first buy, or should I save up for made to measure? I would rather not waste money on something that arrives and does not fit. Anything you wish someone had told you before your first order?