Their proportion to each other should be pleasing and complete a balanced composition. Balance is either symmetrical or asymmetrical.
Asymmetrical balance is random but stable.
This visual equilibrium can be achieved bo grouping several small items to counterbalance a single large item. This also applies in landscape design. Snap a photograph from your main point of view. Then, hold it upside down. If it looks tippy or unstable, the visual weight of the components needs to be corrected to achieve balance.