DefaultFontSizeLabel
When you tape a text without choosing the font size the "Size" combo is left empty (you see a blank field). Changing the DeaultFontSizeLabel gives you the possibility to change this behavior. Example:
FCKConfig.DefaultFontSizeLabel = 'Standard Size' ;
You've chosen to label the "Size" combo 'Standard Size'. When you will type the text you will see that the size label is 'Standard Size'. If you choose a size e.g.'medium' from the combo the label will change as well. If you try to go back to the text written without changing the size you will see that it is labeled 'Standard Size'. So as you can see the "DefaultFontSizeLabel" determines how the normally typed text will be labeled.