Well...it kind of depends on what kind of image you want to make. The fact that it's for Joomla doesn't matter, really.
Why don't you take a look here: http://www.smashingmagazine.com/2008/03/18/adobe-fireworks-tutorials-and-downloads-best-of /
Okay. The image has a gold gradient from top to bottom, but the text repeats in smaller blocks.
First, you need a tiling image of some text, on a transparent background. Then, this image needs to be added as a pattern in Fireworks.
Next, you get to use your new pattern. In your document, create a tall rectangle with a vertical gradient. Make this image the same width as your tiling pattern, so that your resulting background image will also tile. Over that you put a rectangle with no color fill, but fill it with the text pattern. Save this working Fireworks document, then export to use on the Web.
I don't understand the phrase "I need to know how to size the image, possibly with CSS." Background images are the size they are. When this image is set to be the background, then it should be specified to repeat in the horizontal direction (repeat-x).
Here's some info on making a seamless tile:
There have been some recent threads on using gradients, just flip back a page or two (unless my mind's playing tricks on me and they're older).
Hmm... Possibly helpful: