If you mean the size of the first screen that pops up when you click the button, you can do change the size. You should modify file catalog\view\theme\default\template\module\facebooklogin.php on line 55 ,

 

newwindow=window.open(facebookURL,'name','height=680,width=550');

 

The numbers in yellow are the ones that you need to change. Then save and upload if necessary.