Put the following in the BB Code Editor. (6 lines)
[SIZE=2]line1[/SIZE] [SIZE=2]line2[/SIZE] [SIZE=2]line3[/SIZE] [SIZE=2]line4 [/SIZE] [SIZE=2]line5[/SIZE]
It will show you 5 lines.
If you Save, it will be rendered with 6 lines everywhere except in the WYSIWYG editor.
WYSIWYG should render the lines properly.