Resizable Tumblr Chatbox by Ashli Katt

This fairly minimal extension allows you to specify the size of the Tumblr chat-box. It supports editing both the window's size and the size of the message bubbles themselves— though does not support any other features. All settings are static and set through the extension's settings page, you cannot click-and-drag to resize the chat-box.

Disclaimer: This add-on, at least in its current form, relies on seemingly auto-generated Tumblr CSS class names— Tumblr updates may break this plugin but I'll attempt to keep it up-to-date and work on letting the add-on automatically handle it. (Of course, assuming that this is even a concern, and that the class names aren't just static)