Hi Steve,

That's a very good question. The answer to your question is that the Sun appears to be in hydrostatic equilibrium. What this means is that there is a balance between gravitational forces and the pressure of the plasma in the Sun. If the nuclear fusion reactions inside the Sun were to stop, the gravitational forces due to the Sun's huge mass would cause it to collapse in on itself, or shrink in size. If the temperature and pressure inside the Sun were to increase, the Sun would start to expand until it reached a new equilibrium radius. You can learn more about hydrostatic equilibrium and stellar evolution here:

http://imagine.gsfc.nasa....e/know_l2/stars.html

Kris