It could be more aesthetically pleasing (nicer icons, better pop-up window layout, and a pop-up window that isn't 100% tall) but it works! :) Thank you for your work against the dumbing down of Firefox!
Today's update includes a button in the popup to store a custom width and height. If you have specific ideas on icons and layout, you can send them to me using either Github or email (left column of the main page).
Update. I tried to improve the appearance in version 1.8.
