Our sun falls into a classification of star called a "yellow dwarf". It's a fairly small Sun and burns at a pretty middle-of-the-range temperature. In fact there's nothing remarkable or rare or unusual about our Sun at all!
The shape and size it has depends on two factors: the heat from the core's nuclear fusion reacion pushes everything out and the gravitational pull from the core pulls everything in. These two effects (heat out, gravity in) gives the star its size and spherical shape.
In around 4.5 billion years (the sun is halfway through its lifetime) the Sun is going to change. The Sun's main fuel source is Hydrogen, but that will run out and the core will gradually start to shrink. As it shrinks this will concentrate its heat making it hotter, so the "outward" force will start to increase, gradually overpowering its current gravitational pull. The Sun will slowly begin to swell, getting larger and larger as the heat rises.
By 5 billion years time, the heat from the condensed core will be so great that the Sun will have expanded to swallow the inner planets, including the Earth. It will then have changed its classification to what we call a "red giant". Once this happens the Earth is doomed. If we plan on surviving as a species, we're going to have to get going!