Entropy increases as available energy decreases. In other words, the purely natural tendency of things is to move toward chaos, not order, and available energy necessary for work is lost in this process. Eventually, the universe will run down and all life and motion will cease. This is the natural tendency of all things.
Batteries run down, machines break, buildings crumble, roads decay, living things die, etc.
Left to the natural state, all things would eventually cease to function. The universe is not infinitely old because it is not in a state of entropy. If the universe were infinitely old, it would have reached a state of entropy long ago. But, we are not in a state of entropy, therefore, the universe is not infinitely old and must have had a beginning. Because the universe has had a beginning it is not infinite in size.