What I have been doing lately is changing the CodeMirror css files themselves to get it to become larger. In your extensions folder under
CodeMirror\CSS\codemirror.css change the top 2 classes to the following:
/* Necessary so the scrollbar can be absolutely positioned within the wrapper on Lion. */
/* This prevents unwanted scrollbars from showing up on the body and wrapper in IE. */
/* This is needed to prevent an IE bug where the scrolled content
is visible outside of the scrolling box. */
In explanation I changed the height of code-mirror and also the way in which its overflow works to support this extra functionality. I have tested this in Chrome 43 and IE 11 with no bugs found so far.