According to the (conflicting) documentation of TinyMCE, the editor takes on the size of the textarea (or other) element that it replaces. It also says that you can set the
I find the best method to restrict the width tinyMCE 4+ textarea is to wrap the textarea in a new div or span tag. Then apply the width to that. This way you get use percentage widths instead of fixed px. if you want to adjust height edit the rows="15" value.