This is by design:

 

When the user resizes the editor using "+" button, the information is stored in the client side. When the page post backs, the height of editor will be set based on the value of Editor.Height property.

 

For what you want to do, this is my suggestion.

 

1. Create a hidden field

 

2. When page posts back, get the editor runtime height and pass it to the hidden field

 

3. Get the value of hidden field in the server side and pass it to Editor.Height property.

 

 

To get the editor runtime height, you can reference the following code:

    // get the cute editor instance
    var editor1 = document.getElementById('<%=Editor1.ClientID%>');
    
    // Get the editor runtime height
    alert(editor.runtimeStyle.height);