Create a distance measure tool with Create -> Measure Tools -> Distance Tool. Then click twice to create two locator and a node which will determine the distance between them.
Click on the object, shift click one locator and apply a point constrain. Within the Animation Menu go to the option box of Constain -> Point. Make sure Maintain Offset is off and apply. Repeat with the camera and other locator.
Then create a Multiply Divide node. Go into your hypershade (connections preferably). Click on Rendering -> Create Render Node. These might officially be render nodes, but I've never seen them be used for rendering or materials. They are in essence mathematical operations, which will compute faster then expressions because they wont be evaluated every frame, but only when the information has changed. These are the texture nodes he meant.
Anyways, after you click on Create Render Nodes. go into the utilities tab (these were the nodes i spoke of). Click on Multiply Divide.
Then go to Window -> General Editors -> Connection Editor, to hook the different bits up. First get the multiply divide node by going into your outline. First make sure that under Display (upper left of outliner). That DAG objects only is off. Then find the multiply divide node and reload it as a input connection.
Then select your distance node , however you can't use the transform node, but the shape node. Which are two different things, click on the distance node in yourliner, then in order to acces the shape node press down with your arrow keys. Use that as a outgoing connection.
Now connect the distance and put it into input 1 X. Then change the multiply divide node to be a output and put in your object as a input connection. Now select the output X of your multiply divide node to connect to the x, y and z scale's of your object. You can now change the ammount with which your object scales by altering the input 2x attribute of your multiply divide node.
In order to prevent the object from scaling if the camera has gone beyond a certain threshold. Put in a condition node inbetween the distance node and the multiply divide node. Simply say that the input has to be equal or less than the ammount specified, if it is false and thus more. Then make it feed the threshold distance.