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:
.CodeMirror {
line-height: 1em;
font-family: monospace;
height: 100%;
/* Necessary so the scrollbar can be absolutely positioned within the wrapper on Lion. */
position: relative;
/* This prevents unwanted scrollbars from showing up on the body and wrapper in IE. */
overflow: hidden;
}
.CodeMirror-scroll {
overflow: auto;
height: 100%;
/* This is needed to prevent an IE[67] bug where the scrolled content
is visible outside of the scrolling box. */
position: relative;
outline: none;
}
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.