Sometimes tree planting occurs on a whim, and the resulting benefits or functions are simply a surprise. Just think about how much more you can get from your trees if you consider what functions you would like for them to fulfill!
What are your underlying purposes for planting a tree? If you had not considered planting for a reason other than improving the appearance of your yard, think about other benefits you may accomplish with different placement, shapes, or sizes of trees. Different species of trees come with different forms, heights, and spreads, which determine the functions they are best suited for and the spaces where they fit in the best.
In reference to tree size, keep in mind that fast-growing trees are normally weak-wooded as well, not to mention short-lived. Although it requires patience to watch a slow-growing tree increase in size, the rewards of planting a slow-growing tree are highly valued.