About:

By default the 'Show sizes' tool is not in use.

This script automatically appends the required tag in the URL when it finds it absent and also disables the 'Tools' panel.

Changelog:

v1.0: Initial release.
v1.0.1: Fixed a minor bug.
v1.1: Shortened the code.
v1.1.2: Script now automatically hides the 'Tools' panel. (if you don't want this then comment out the part below the if condition)
v1.2: Fixed incorrect results caused while searching different sizes for a uploaded image.
v1.2.1: Updated description.

Other Stuff:

This was my first script. Please leave a constructive feedback. :)

Also check out my other script(s) if you are interested:
Favstar Pro - Best Tweet Unlocker