Your initial Fireworks document size depends on what you're designing. I take it from your question that you mean a Web site layout, but Fireworks can be used for other graphics as well, in which case your canvas size might be much smaller.
One word of caution. While it's very tempting to design in Fireworks and export the code to import into Dreamweaver, the code you get from Fireworks is fragile and difficult to work with. You are much better off designing in Fireworks, slicing your design and exporting the graphics you need, then building your HTML in Dreamweaver (which is specifically intended for that task and really good at it).
But, your question. How wide do you make your site design and consequently your Fireworks document? Any width you like. Here are a couple of articles that discuss the point:
When you design a site, you need to decide if you will have a fixed layout (the content is a fixed number of pixels wide for every visitor) or a fluid layout (the content expands depending on the width of the browser window). If a fixed layout, then you need to consider your visitor base and what type of monitor they are likely to have. Do you go for a wider layout, or a narrower one?
Let's suppose you decide on a fixed layout. Do you make your Fireworks document that width? Well...maybe. If you have a fixed-width layout, you probably want to have some kind of background, maybe a solid color, maybe a tiling image, maybe a large image (as Adobe has for this forum). You might want to include that in your Fireworks document so that you can see how your page might look in a browser. You might want your layout centered or left-aligned in the browser, and having a larger canvas will give you a feeling for how your design will look.
If you decide on a fluid layout, you might choose to have percentage margins on either side of your layout. In that case, you would still design in FW with a fixed layout, maybe even with a canvas larger than your place-holding width. As you create your design in FW, you'd have to keep in mind the fluidity you intend. You would then build your fluid layout in Dreamweaver. As with the fixed-width scenario, you may or may not decide to have your FW document wider than your intended layout, so that you could see any background you want to have.