http://www.freebsd.org/doc/en_US.ISO.../x-config.html
You may have seen this all ready,but the info you need is there. Please see "modes" your defalt mode is set too high i.e. "1024x768" or "1900x1600"
you want a lower mode "800x600" or "640x480"
This is a part of XFree86 that can be hard to understand:
1 there are many ways to configure XFree86
2 XFree86 has too many names i.e. X11 XF86 X11R6(they are all XFree86)
I don't run freeBSD so I don't know where it looks for the XFree86config file or I would point you at it with a path(/ect/X11/XF86config) ,but regardless of the path to the config file that is the file you need to edit.
When you ran the XF86config it should tell you or ask were to save it. i.e.
/usr/YOURUSERNAME/Xfx6config.
you can edit that file with a text editor and remove the Modes that are to high and add the modes you want.
I'd bet freeBSD has a GUI for control of screen size, but I can't point you at it.
Don't be put off by this. Configuring XWindows can be hard because of all the video card monitor combo's out there
1 Find out what file X11 is using as it's configuration file and where it is
2 Edit the MODES "1024x768"
to read "800x600"
3 save the file
4 restart X(KDE)
or
1 Run the XF86 configuration tool you have run
2 when you are asked to select your modes don't select "1024x768" or what ever mode you don't want or don't want to use,deselect them if they are selected for you
3 if you are asked for a defalt mode choose "640x480" or "800x600"