Iveth,
We can help you force the width if you want to. We won't force it to that width though for everyone, because there are users who are using it as it is right now in sidebars and other areas where it has to resize to the content.
If you want to force it yourself (to any width), simply edit (or create a template override) for /modules/mod_jfbcinvite/tmpl/default.php. At line 42, you'll see a div with a width parameter specified as "$closedWidth". Simply remove the PHP variable there, and set it to "940px" or whatever size you'd like to use.
Hopefully this will work for you, cause we'd really like to see you happy with the module. If you need more assistance, definitely let us know. If there's a generic solution we can implement in the JFBCInvite module so it works for everyone, trust me, we're all for it. There are some updates that Facebook has released lately that we're investigating for multi-friend selectors (which is what the Invite module uses), but from initial testing, it doesn't fix all the issues we've seen.
Thanks again for your feedback and patience, and good luck!