You all saw it, the plugin acted buggy these days
First it didn’t work at all.
And then it worked but showed a debug line at the top of the screen.
Now it’s all fixed, so there is the version 0.6.2
With the fullsize in version 1.1.1 from the official website : http://www.addfullsize.com/
You can change all properties in the wordpress options.
Sorry for the inconvenience, and enjoy this great plugin !