Always make sure they fit you in the neck and sleeves which is very important at the end. You should be able to fit two fingers comfortably between your neck and collar and when you turn your head the collar should stay still.
The cuff should end where your thumb and wrist meet. Everything else depends on the cut of the shirt and can be altered by a tailor (or yourself) to find your perfect fit through the sides.
Always is necessary to have shirts in 3 colors in your wardrobe, one for sure is the white shirt, so you can wear more smart and casual, blue shirt the same but is more casual at the end and then, you should have one jeans shirt, totally casual and sport.