The Editor Add-on CSS Package only allows a text area to be resized vertically:
textarea {
 resize: vertical;
}
There is no explanation anywhere concerning why Google doesn't want you to allow a textarea to be resized horizontally.
I've got a problem with my UI elements not expanding to the full width of the dialog box. People need to be able to compose an email in my add-on. They don't want to compose an email in a little tiny box, that won't expand horizontally.
I've downloaded the Editor Add-on CSS Package, and I'm going to use templated HTML to include it in the HTML, and comment out the textarea resize section.