A shirt "off the rack" is rarely perfect, It has either too much material on the shoulders, in the waist or length, but is perfect in the other area. Here comes in the wizard of fashion: tailor. It's amazing what can be made with those too big pieces that are laying around. And when buying a new shirt, we should be thinking about a tailor and what can be done to make it perfect. That's why you should always check if it's right in the shoulders and collar, because it's the hardest part to change.
Shoulders - Make sure that seams end just at the point where your shoulders start to curve down. If the shirt is too wide, the top of your body would look sloppy and give impression of somebody that is "drowning" in his own clothes. On the other hand, if the shoulders seams are too high, the fabric will probably be too tight to wear comfortably and will create some ugly stretches.
Collar should just gently brush on your neck, not clamp on it. Button it up and try to put two fingers between the fabric and your skin - they should fit easily. But remember, that if three fingers can go inside, it might be too loose. Other way that you can check if the collar is too tight is looking in the mirror and turning your head left and right - if the collar is moving with you, then look for a larger one.
Length is important when tucking it in as well as casually leaving it out. Too long and it might look like a dress or pajama and make you look shorter than you are. When tucked, it will create extensive bagginess near the belt and add extra roundness into your figure, which is something that we should avoid at all times. Make shure that the fabric doesn't some out during usual movement (walking, sitting down, dancing) when tucked in.
Sleeves and cuffs - Clothing designers usually associate sleeve length with collar size, there are usually two lengths per collar. There might be a situation, where all the shirts with a perfect collar will have non fitting sleeve. There would be no way of avoiding service from an experienced tailor. There is a little trick that you can do to check the size: Stand up straight with your arms hanging down your body, cuffs should end on your wristbones. Then, raise your arms and check how much the sleeve gets pulled back, if it's more than an inch, then you need to get them tailored. Cuffs should be tight enough to lay on your wrist bones, but loose enought to have some space for a watch.