Even men have to follow fashion rules, and certain guidelines apply as to whether to tuck in a shirt or leave it untucked. Most fashion dos and don'ts regarding shirts are based on the height and weight of the man, others on the occasion for which the shirt is being worn. One rule that applies to everyone: never tuck any shirt into pants or shorts with an elastic waist.
Short in Stature
If you are a short man with proportioned legs and chest, always tuck in a dress shirt. Tucking your shirt in will make you appear taller and will keep you from looking like you're wearing one of your dad's dress shirts. Because it's a neater and more professional look, it will also make you seem older and not like a "short kid." A striped shirt that is tucked in will give you visual height. Some casual shirts seem long enough to have been made for basketball players, with lots of added length on the bottom. Leaving a casual shirt, such as a polo shirt, untucked, will overwhelm you and make your legs seem shorter. If you are short man with longer legs and a short torso, tuck your shirt in and wear your trousers a little lower so you don't emphasize the shortness of your chest. This visual trick will make you appear more in proportion.
The Athletic Body
If you work out and have the V-shaped torso that most men crave, tucking in your shirt will show it off. If your dress shirt is a little loose in the sides, take it to a tailor and have it taken in. That new seam, with the combination of tucking in your shirt, will show off your narrow waist. If you are wearing a casual shirt, don't be afraid to choose one that is a a little more clingy. Tuck it in and everyone will notice all the hours you spend at the gym.
If you have a small frame and feel too "skinny," to tuck or not tuck depends on the shirt. If you are wearing a shirt with a slim or tapered fit, tucking it in will only emphasize your thin frame. If you want to tuck your shirt in, wear a shirt with a little more fullness to give you the look of more weight. Go for a slim fit in your pants, but not as slim as skinny jeans. This will give you an overall balance. Always tuck in a dress shirt when necessary, but wear one that is a little fuller if you don't want to draw attention to your thinness.
A barrel chested man with a thicker midsection can feel free to either tuck or not. Leaving a shirt untucked can camouflage a thick waist. Don't wear a shirt that clings to the stomach alone and nowhere else. Make sure the shirt drops from the shoulders and is loose enough so there is no cling. If you decide to tuck in a shirt, don't push the shirt in tight. Blouse it a little so it pulls away from the stomach.
Heavy in the Tummy
A man with weight in his stomach should keep most of his shirts untucked. A shirt that is loose will camouflage extra weight. Make sure it's not too long or you may end up looking like you're wearing a dress. Most polo shirts are made with a little more room in the midsection, so you're usually not in danger of having one that is too tight. If you're wearing a suit, or you must wear your shirt tucked in, make sure it is not too clingy in the stomach area. A striped shirt will elongate your midsection.