It's pretty simple.
1. Add the mouse object and the sprite you want to use as a button.
2. In the event editor add two events:
+System | Every Tick
--Sprite | Set Size to (32,32)
+Mouse | Cursor is over Sprite
--Sprite | Set Size to (50,50)
The first event is for when the mouse is not over the sprite and the second is for when it is. You can do whatever effect you want for the rollover but as an example I just used size.