For those having trouble: try alt+cmd+m instead - that worked for me. Default presets are kinda limited, but looks easy enough to add your own. Very handy extension as it also manages height, not just width. Thanks Carlos!
Sorry Carlos. I'm unable to get this to work either.
Clicking "Install Presets" does nothing, nor does CTRL + SHIFT + M. I would have sent you an email to trouble-shoot prior to leaving a 1-start review, but the email link on your homepage is a 404 (not working either).
Hopefully you can get this working. Looks promising!