Not always, i think its like judging the doughnut by its hole. Its part of it's make up but don't let the doughnuts hole detract you from the whole of the doughtnut.
The size of the tree is important for it's own micro-climate. There is a whole world of life's interactions going on in any one particular tree. The bigger the tree, the more that is going on, generally. But even the small trees are essential to how i value them. It is the size of the world population of trees that has a more direct effect on the biosphere.
Sometimes. Larger or older trees can provide more habitat, and of course, they produce more oxygen. They also have stronger and more extensive root systems. And a tree has to be mature before it can reproduce.