You say:

 

"I suggest you use a hidden field in my answer."

 

How am I supposed to use a hidden field, Adam? The field can only be populated if there is a trigger event that we can code into to assign the height into the hidden field which we can then read server-side on postback.

 

How do we do that?

 

Take a minute and show me the code that populates the hidden field. The only way this can be done is by a client-side event. So what is the event that is called when the user clicks the "+" or "-" button.

 

Please expand your answer to demonstrate how this can be achieved.