Trimming the Canvas
The final step is to make the document size smaller. You need to trim the excess white area of the canvas around the image to make the image size as small as possible. You could use the Crop tool, but the easiest method is to use the built-in trimming feature.
Choose Modify > Canvas > Trim Canvas, and the work is done for you.
Fireworks makes the canvas as small as it can without cutting off any object. Fireworks knows the exact size and placement of all objects on the page, even if they are not visible. Soft edges such as drop shadows and glows are even accounted for and preserved on the canvas.
Save your file.