There seems to be an issue in EL6 virtual desktops where the font size becomes large when there is a large display resolution.  For example, the font size in the GNOME panel and the icon names are too large. 

If I start an EL6 virtual desktop and then open the console in full screen mode with a display resolution of 1920x1200, the fonts are too large.

If I then restart the virtual desktop in Windowed mode, it will default to a display resolution of 1024x768.  The fonts now appear to be the correct size.  If I then press Shift-F11 to put the virtual desktop into Fullscreen Mode, the display resolution changes to 1920x1200 and the fonts are still correctly sized.

You can see the Dots Per Inch for the display by running the xdpyinfo command.  When an EL6 virtual desktop boots up in windowed mode, the resolution defaults to 1024x768 and the DPI is 96x96.  This seems to indicate a display size of 10.67" x 8.0".  If the display resolution is adjusted to 1920x1200, the DPI is 180x150.  This indicates that Xorg still is using a display size is 10.67" x 8.00", even though the display resolution is much higher. 

It seems to me that the Spice client should be able to pass the display size to spice-vdagent along with the display resolution.  Then, Xorg should allow the display size to be dynamically adjusted.

I have found a public Bugzilla report which seems to be similar to this issue.