Originally Posted by blackout23
It should work in theory without the panning part, but in practice the mouse will be stuck in the 1440x900 part of the picture, adding the panning option fixes it. Make sure to set panning back when you switch to the original resolution:
xrandr --output DVI-I-1 --mode 1440x900 --scale-from 5120x3200 --panning 5120x3200
Not sure why you'd want to do that though, it won't look good.
xrandr --output DVI-I-1 --mode 1440x900 --scale 1x1 --panning 1440x900