If we're talking 72ppi, then your graphic is around 14" square. You could lay it out 600x800 horizontal and "Save for Web" as either a .gif or a .jpg ( depending on the artwork ). Try not to scale anything in Dreamweaver.
Just in case a visual will help you, here's a quick tutorial about scaling for web:
1) Go to File > Save for Web
2) This will open a dialog with lots of options for saving.
The area marked as "A" is where you would choose the file type for your image, which, as John said, would be a .gif or a .jpg, or a .png if your document has transparency.
The section Marked "B" is where you can easily resize your document. Unless you click on the chainlink icon beside the dimension boxes, your image will automatically retain its proportions when you enter a new value for height or width. Also be sure to tick the "Clip to Artboard" option so you don't get any extra objects that may be lingering in your workspace.
I would also agree with John when he says to recompose in an artboard in the desired dimensions of your final image intended for web. And you're exactly right about the multiple artboards for different devices. In the screen shot below, I have an artboard in the dimensions of your original document (1022x1021) and one for the dimensions of an iPhone screen. This makes it really easy to modify compositions based on the sizes and proportions of different devices.
Here is a link about preparing graphics for multiple devices: http://codebycoffee.com/2010/12/08/preparing-graphics-for-multiple-mobile-platforms/
I hope this helps! If you need more web assistance, don't hesitate to post in the Dreamweaver forum!
thanks for these answers, do you know where I can find all the standard dimensions in pixels for all devices even monitors?
It's always painful when you finish an art and you gotta find all the right canvas dimensions to fit it in
may be i should create a template for this.
hi john whats the formula you use to calculate that?