I use tags in my module titles,
e.g.
Categories.
I specify the span\'s background color/imag
You can't specify the width of an element with display inline. You could put something in it like a non-breaking space ( ) and then set the padding to give it some more width but you can't control it directly.
You could use display inline-block but that isn't widely supported.
A real hack would be to put an image inside and then set the width of that. Something like a transparent 1 pixel GIF. Not the recommended approach however.