MW94,
It sounds as if you haven't set anything as a background. In VSD, there are a few different ways to set your background; there's Page background (color, gradient or image), Continue Page background (takes that same color, etc. and extends it beyond the page edges to fill the browser window, and then there's a separate browser background (color, gradient or image) that can be different from the Page background.
To see how this works... Go to Page Properties and pick a color for your background. Now go to Settings (top menu), and under Background Fill, choose a different color to show beyond the page.
If you're going to use images, be aware that they will typically repeat horizontally and vertically to fill whatever space they're placed in. (But that's a whole new subject

)
Keep your page size at something reasonable so viewers won't have to scroll back and forth as well as vertically (1024px wide is good for a variety of screen sizes).