I made a quick layout image for some of the more common CSS elements in a tiddler.
NOTE: The .tc-titlebar includes the whole width of .tc-title and the area with the tiddler buttons.
It is here at Dropbox.
In Firefox, you can go to Tools, Web Developer, Inspector, then use your cursor to ID the elements. Note that to control some spacing you might have to change the padding, or margin. I did not see any border CSS used.