The below image shows a good way to go about this:
<img src="https://dl.dropbox.com/u/917406/releaseButtonExample.png" border="0" />
The first condition shows a standard "Is touching object" event, but inverted. This means that if the object is not being touched, then the condition will be true. To invert a condition, right click on it and press invert.
The second condition indicates that if the condition(s) above it are true, then the event will only run the first time that the conditions are true - however, if the conditions become false and then return to true, the event will run again.
Initialwidth simply implies the initial width that the object took.
Basically, it boils down to this: the first time the button isn't being touched, change the width of the object to the original.
I hope this helps you out.