Is there any way to change the size of arrow heads?

Yes, you can change the size, and direction, of arrow heads. Double-click on your line or arrow to bring up the Format Object dialog. From there you can choose the size of the arrow head and also the style (you can have capped lines as a type of arrow). If you need more choices, try SmartDraw. You can embed a SmartDraw object on a Prism graph. SmartDraw offers lots of choices for drawing arrowheads, as well as lots of other drawing tools and a nifty way to create tables..