In case you are using the what you see is what you get editor, which is a more visual editor that can be activated from your forum profile,
you might encounter font size issue, it is a bug. Not fixed yet.
I had this issue many times and I have WYSIWYG settings enabled in my settings from start.
Only way I found to temporary ''fix'' this is to select entire text if my content and select font size 10, than post it.
This is not perfect solution and sometimes I would get additional text that can't be erased (SIZE, etc) unless I delete everything.