Does Fireworks have a similar utility to Illustrator's 'Save for Web and Devices' where you can preview a JEPG or PNG while you vary the compression and filesize?
The File->Image preview does exactly that.
You can also go to the 2-Up/4-Up view to toggle the optimize settings and then see the file sizes.
You don't even have to take that step. Simply change the view from original to Preview, 2-Up or 4-Up in the main interface and change the settings in the Optimize panel. Save yourself time and mouse-clicks