From UbuntuForums.org:
Re: How to precisely resize windows in XFCE?
xdotool will allow you to resize windows on the fly. For example, to resize an existing Mousepad window to 400x400 pixels:
xdotool windowsize $(xdotool search --name "Mousepad" | tail -1) 400 400
If you want to start a program with a specific size, you can use the --geometry or -g parameters if the application supports them, or use a tool like devilspie.
Running the xdotool command worked perfectly. I just substituted Mousepad with whatever the name of the application was.
If you want to select the target window interactively, you can combine the two xdotool window commands windowsize and selectwindow:
xdotool selectwindow windowsize 800 600