The body band around your torso should fit firmly yet comfortably. It should be in a level position round the body and not ride up the back. The under wires at the front should feel comfortable and sit firmly against the chest wall. It should not dig into your body or cause any bulges.
The cups of the bra should be big enough to fully encase your breasts.
The bra should create a smooth line around your breasts regardless of the style of the bra (balconnet, 3/4 cup etc…)