You mean you want to make it smaller as it plays?
Make a new event, "Every tick", and "Explosion - 'Exploding' animation is playing" with the conditions "Explosion - Set width to Explosion.Width-1" and Explosion - Set height to Explosion.Height-1".
You can change the "-1" to be bigger to make it shrink faster, or smaller to make it shrink slower. It might need to be smaller -- getting a pixel smaller every tick will make it shrink really fast.
Hope that helps. <img src="smileys/smiley1.gif" border="0" align="middle" />