The height is totally defined by Facebook and depends on how many comments you have set to display and how many comments have been made in the box. If you set it to 5, but no comments have been made, the box will decrease it's overall size. There are times when a page has 0 comments that a white, blank, box will display beneath the comment input area. This is a bug with the Comment box that Facebook is aware of, but they will have to fix on their end.
Hope that helps, but if you need anything else, let us know.
Where is the height of jfbccomments changed
