WELCOME TO ALICE! haha, this is a major problem in alice as collision detection is not a very straight forward process.
You seem to be on the same idea as I was, "using half the width of an object" this is a good start.
Several problems do exist however:
One - The process you described will only really work properly on perfect sphere's, if you object is asymetrical obviously some parts are going to be farther away from the center than others.
Two - using this method assumes that alice will correctly find the center of the volume of the object, often it seems to calculate distance from other points such as the center of the farthest part of the back of an object.
Three - when using move commands such as move 3 meters forward, your command attempts at collision detection will not come into effect until the move is completed.
Here is a game I made in which i attempt to use some collision detection, I quickly gave up and moved on to other issues.
http://alice.org/community/showthread.php?t=2631