When you try on a suit, try to stand in your “Natural stance” this would be how you stand the most often, just let your body stand natural, your muscle memory will help you with this.
You don’t want the suit to be too loose or too tight, pay extra attention to where your major joints are (Elbows, Shoulders, Hips, Knees, Etc.) If the clothes are too tight you won’t be able to use your full range of motion. If the clothes are too loose, they will be baggy and bunch up in those main key places. You also don’t want the sleeves to pass your wrist too far or the pants to be so long the fabric goes underneath your shoes.