You are appending the elements to the form $("#formular") and that form already has one child, the div with the id of firstelement. So after you clone the fieldset and append it, it is the second element which is what $("#formular").children().size(); is telling you.
If you want to get a count specifically of the fieldsets then change it to this: $("#formular").children("fieldset").size()
If it isn't fieldset specific then just decrement your size by one and use that.
If there will be other elements and they'll need separate indexes you'll need to use the index method
http://api.jquery.com/index/ to determine the position of each one after you add it, or you could simply create a counter and increment it for each type of element.
Hope that helps