If you create a web-page that is bigger than your visitor's screen resolution, (in terms of pixels) their browser will put scroll bars on the browser window, both horizontal and vertical, if required.
The width of your final web-page is determined by where you place objects on your workpage. In particular, if you place something 1000 pixels from the left margin, you will see a horizontal scroll bar when you view that finished page at 800x600. As will your visitor.
You might look at your site statistics and see that most of your visitors are 1024x768, so you might be tempted to use that. But not so fast! Are your visitors browsing full screen or at somewhat less than full screen? I am in the latter category, so even though my screen resolution is 1024x768, I am quite happy to view an 800x600 page -- in fact it nicely fits the window I normally use.
Even at 800x600, I think it a good idea to keep inside the 800 by 50 to 100px on each side. The reason is that a window in an 800 screen actually has somewhat less than 800 actual width. There are the window borders, the space for the vertical scroll bar, any windows docked to the left-hand side of the browser window (like favorites) and even the Window task bar (the one with the start button). Even though the default is for it to be along the bottom of the screen, you can drag it to place it up the edge of the screen.
So I suggest even if you conclude that your visitors are mostly using greater than 800x600, that is still a good resolution to use.
You can see the effect in preview by setting your browser to almost full width then dragging in the right-hand border of your browser. If your screen resolution is high enough, you will see the white margins at the edge of the page change in size. If your resolution is 1024x768 or better, you can see the effect by resizing this page.
For this centering to work properly, your workpage contents should to be centered around the center line of your design width.
If you put extra objects asymmetrically outside the design width, you will upset the centering. To help you center things exactly, don't forget the center button on the bottom toolbar.
I suggest don't worry about trying to eliminate a vertical scrollbar -- much depends on the visitor's computer and browser settings -- e.g. how big the taskbar and toolbars are.
Horizontal Positioning and Sizing
Using horizontal positioning and sizing is incompatible with the horizontal page centering that I recommend, so I suggest you don't use it. It's rather tricky to get right in any case -- so why make things more difficult? For more on this, see the tutorials that are included with SiteSpinner.