Let me suggest a different approach. This is your goal as indicated:
I want the input forms to be the same height as the buttons on the right.
In addition, there is a condition for allowing text changes, as indicated:
can still be used if the text size was larger than this height
Knowing that my suggestion is to base height on EM. Use EM to determine the height of the input container and button, then set the input and button heights to 100%. Thus, when the user resets their font size (from the smallest to the largest), the container will grow and shrink, and the enter / button will grow and shrink with them.
I mocked a simple example at the following URL: http://jsbin.com/oguze5/2/edit
Things will need to be changed for styling purposes, but the general idea / concept sounds pretty good.
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