I would like to make the visible workspace of my desktop fill up less space than the entire monitor while keeping image fidelity constant. Presumably, any solution would leave a black border around the new, smaller main window. I am pretty sure this involves making some tweaks to xorg.conf (like changing the DisplaySize and X and Y position of the root window or something), but nothing has worked for me. I am running Debian "Squeeze".
Look into the Xrandr man page xrandr. If I am reading into your question correctly, I have achieved this on a few occasions by messing different options (see below). It can be done per-session, without having to change xorg.conf
From the man page:
After you list all outputs and available modes, and depending on what modes your Video Display supports, you can run the following for example. The man page lists other examples and features such as panning, rotating, hsync &c.
Hope this helps.
Launch a second X server, using XNest, and position it within the real root window. This has several advantages over messing with xorg.conf files: