xdotool search --onlyvisible --name firefox
xdotool windowsize $WINDOW_ID_GOES_HERE $WIDTH $HEIGHT
xdotool windowmove $WINDOW_ID_GOES_HERE $X $Y
For example, if the window id for firefox is 123 you would do this:
xdotool windowsize 123 800 600
xdotool search --onlyvisible --name firefox
xdotool windowsize $WINDOW_ID_GOES_HERE $WIDTH $HEIGHT
xdotool windowmove $WINDOW_ID_GOES_HERE $X $Y
For example, if the window id for firefox is 123 you would do this:
xdotool windowsize 123 800 600