1. SHOULDER LINE-UP: One of the easiest ways to determine if a shirt fits correctly is through the 'shoulder' line-up. The shoulder seam should sit in-line with acromion [shoulder bone i.e. the bone sticking out at the shoulder]. If the shirt falls more than 0.5-1 inches outside of this, the shirt is too big If the shirt falls more than 0.5-1 inches inside of this, the shirt is too tight. The right balance is key for the best visual to be created at the shoulders.
2. ARMS: When aiming to dress for a formal occasion, the clothing should appear to be tailored and fitted. One of the easiest ways diversions happen on a dress shirt is through arm segment of the shirt. Here the shirt must wrap snugly (not tight!) in order to present a tailored aesthetic whilst also providing structure to your arms the easiest way this can be achieved is simply by purchasing slim/ tailored fitted shirts, or you could simply fold your sleeves.
Example for muscle fit t shirt that make your arms look bigger: https://extended-alpha.com/products/muscle-fit-t-shirt-1
3. TORSO: The final component of a well fitted shirt is the Torso. With the torso the largest section a shirt covers, this is the make or break when it comes to shirts. Again you want this region to be tailored and fitted, with a slimmer fitting (Not tight!) in the chest/ shoulder region and with a tapered lower to the stomach. This helps create the illusion of broader shoulders and a tapered waist.
SHIRT STAYS: If you shirt is too long/ too baggy or if you just want to improve the overall aesthetic. A very good investment is a shirt stay. This is a tool used to keep your shirt tucked all day, it's worn around the legs with clip extensions to clip onto a shirt holding it in place This helps to eliminate excess fabric creating a streamlined, fitted look throughout the shirt.
SLEEVE ROLLING: This is by no means a substitute for a well fitted shirt, but this can help create the illusion of a better fitting shirt particularly in the arm region. To roll the sleeve correctly, it's important that the 'roll' is correctly tucked and is accumulative of the fabric before that [detailed link in the description below] This helps 'tighten' the shirt at the sleeve/ elbow, creating the illusion of a better fitting shirt but also allowing for the much loved exposure of the forearms.