The first and most important design principle pertaining to size is to select plants whose mature height and spread will fit the intended space and serve the intended function. For example, plants that are to serve as walls in the garden should grow tall enough to provide privacy while keeping within their allotted space.
The size of young shrubs and trees can be deceptive; it is often difficult to image how tall and wide they will grow. This is especially important with trees, which will continue to grow larger for decades.
Make a conscious effort to choose plants of varying height, which will make the garden much more interesting than one in which the plants are all approximately the same height. However, avoid extreme changes; try stair-step planting, placing the tallest plants in the rear and the shortest in front. Of course, make sure you don’t inadvertently screen off a desired view by choosing plants that will grow too tall.
Newly landscaped gardens, if planted with the correct spacing between plants, naturally look sparse, with more soil showing than plants. In the eagerness for a lush garden, many gardeners space the young plants closer together than recommended. They soon find that they must deal with overcrowded plants that are competing for light, water, and nutrients. Although it may seem a nuisance to look up the mature size of each plant and allow for it in your plan, it is worth the effort.
If you can’t tolerate the sparse look for a few years, consider filling out the garden with temporary plants. These might be a wildflower mix from a can of seed, beds of annuals to add color, or fast-growing trees to provide shade until the permanent trees grow tall enough. Be very disciplined about removing the temporary trees after they have fulfilled their function, though, or the landscape will become cluttered and the permanent plantings will be damaged by crowded conditions.