I fixed it leaving the "Comments Width" blank. Not expected because it says "(required)". Now FB comments window has the perfect size on desktop and mobile.
But I have a new problem now, the plugin is in english, the "Language" is not working after leaving the "Comments Width" blank O_o I suppose this is because my theme has fb comment integrated, but it is deactivated for the post I am testing, the thing is this plugin is actually working and adding comments to wordpress.
EDIT: Now, whatever I do, there is no way to change language now, even if I add the "Comments Width" again.
EDIT2: I changed theme to default one, and the plugin works normally, even the "Comments Width" blank trick is working, and language too. Now I need to "guess" how to enable my (premium) theme without it interfering with this plugin.