you can use
xrandr to change the resolution of your desktop. if you cut your resolution by half, that will have the same effect as doubling the pixel size.
xrandr is the X-Windows Resize And Rotate tool.
running it with no options will give you the list of supported resolutions:
Screen 0: minimum 640 x 480, current 1440 x 878, maximum 1440 x 900
default connected 1440x878+0+0 0mm x 0mm
1440x900 60.0 2.0
You can use it to change to one of the supported resolutions by passing the new size:
bash-$ xrandr --size 800x600
I don't know of any way to force a single application, or a single X window to be displayed at a different resolution than the root window, though.
You can use the
xdpyinfo command to see if your display support dynamic resizing.
Run it with no arguments, and you're looking to see if the
RANDR extension is there:
bash-$ xdpyinfo | more
<snip some stuff>
number of extensions: 23
Generic Event Extension
If you see
RANDR in that list, then your display supports using
xrandr. You may need to install the package that provides
xrandr, and which package that is and how to install it would depend on what distro/OS you are using.