Script Summary: Play Minecraft in a popup at any size you choose!
This script will add a button underneath the game that says "Open Popup Client"
1. Click "Open Popup Client" and a pop up opens
2. Choose the size you want the game to run at by resizing the window (You can maximize it or hit F11 to go to full screen)
3. Hit the start button when you're happy with the size.
You will need to use Firefox with the Greasemonkey addon.