FIA underwear should feel close-fitting without compressing: it is your first protection layer and also affects breathability during a long stint.
Measure chest, waist and hips with the garment snug to the body, then compare with the product size chart. If you are between sizes, favour the one that keeps shoulders and hips free to move.
A good base layer reduces chafing, improves thermal comfort and helps the outer suit perform as intended.