This depends on how you disable the editor. Generally, it should not change its dimensions but some page configuration or CSS rule may be affecting it. What I can advise at this point, without having any information about this case or having seen this issue, is that you inspect the rendered HTML of the disabled editor, find the element that is being too wide and add a CSS rule that will keep it to the desired size. If this does not help I advise that you post the code that leads to the problem so we can examine it.
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