Hi,
You should be able to refer this library (JS and CSS files) in your CEWP (or custom area in JSLink setup) and address the textarea like this:
$(function() {
$("#dffs_textAreaFieldInternalName").find("textarea").resizable({
handles: "se"
});
});
Change “textAreaFieldInternalName” for your fields internal name.
Please note that this will work for plain text fields only.
Alexander