The band of a newly purchased bra should fit well on the loosest setting (the outer-most hook setting). A band that cuts in uncomfortably may be too small. A band that rides up the back is too large.
The bra band supports most of the breast weight, so the straps should not be tightened up too much. They shouldn’t dig into the shoulders uncomfortably.
When putting the bra on, bend over forward and scoop forward all the breast tissue, settling it into the cups. We are not trying to fit the armpit/side tissue (Tail of Spence) into the bra, but only actual breast tissue. If the cups are wrinkled, they may be too large or the wrong shape for you. If you are overflowing the cups, they are probably too small.
The underwire should sit comfortably around the perimeter of the breast tissue, not cutting into the breasts, nor extending beyond.
The centre front of the bra is called the gore. If the bra is wireless, the gore will likely not sit flat against the body. If the bra is wired and fits correctly, the gore should sit flat against the sternum.
It is possible that a bra can be the right size but the gore folds over itself or the wires can’t lie flat because the breasts are too close together. In this case, you might choose a bra with a lower gore, like a plunge style.