Change width (cols) of text area?

Changes like these usually need to go into your theme stylesheet or in some cases into a custom CSS setting in the appearances menu.

To find out what CSS rules are affecting the elements, I suggest you try FireBug on Firefox or the developer tools built into other browsers like Chrome or Internet Explorer. The idea is you’ll need to create new CSS rules that set the element dimensions how you want them to be.

When I inspect the element I get the id pdb-story and the classes text-area (also required-field but I know that’s not it). I’ve looked through all the plugin’s css files and can’t find either. But I don’t think css would do it because the rows and cols for the text area are explicitly written inline, which I believe would supersede any linked css, right?

No, CSS will override that. Rows and Cols are just a special way to determine the dimensions of a text area. Height and Width CSS rules will be applied and will override the dimesions set in the element as long as they are properly qualified.