The line numbers use the same size font as the TextArea.
It is very simply to use, simply set the skinClass property in mxml like this:
If you want a horizontal scroll bar on the TextArea, then set lineBreak="explicit" and the horizontal scrollbar will appear.
Here is an example of it in action (right click to view source).
It would be very easy to modify this skin to make it resizable by adding a resize handle in the bottom right corner. Look at the source code from my previous blog post on Resizable Controls, specifically the flex.utils.spark.resize.ResizableTextAreaSkin class, it uses a custom skin for the Scroller, which adds the resize handle.