In this first post the information comes from a thread I first issued on the slax forum. The link is here:
http://old.slax.org/forum.php?action=vi ... ostid30879

General instructions.
Porteus usually provides the maximum screen size possible for the screens attached to your computer. Xrandr commands easily fix dual display problems. To help with your display or screen difficulties please provide make and model of your screen and the output in command line of:
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xrandr
1.0 Fixing a single screen resolution xrandr.
See the experience of @Petr.kx, who was caught with a resolution under the one that could be attained with his display:
http://www.slax.org/forum.php?action=vi ... ostid37773

Here Petr.kx wanted to add a resolution 1400x900 with a frequency of 60 that was not provided stock by the xorg.conf setting:
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root@slax:~# gtf 1400 900 60
# 1400x900 @ 60.00 Hz (GTF) hsync: 55.92 kHz; pclk: 104.23 MHz
Modeline "1400x900_60.00" 104.23 1400 1480 1632 1864 900 901 904 932 -HSync +Vsync
root@slax:~# xrandr --newmode "1400x900_60.00" 104.23 1400 1480 1632 1864 900 901 904 932 -HSync +Vsync
root@slax:~# xrandr --addmode VGA "1400x900_60.00"
http://manual.aptosid.com/en/hw-dev-mon-en.htm
3.0 Adjusting colors and brightness with xrandr: the gamma specification (this section is experimental, I am not sure that what I proposed is the real thing. However, the links are good and merit in depth reading and testing):
http://unix.stackexchange.com/questions ... -and-color
http://www.normankoren.com/makingfineprints1A.html
http://www.normankoren.com/makingfineprints1A.html
An ideal gamma configuration 1.05:1.15:1.136:
http://archive09.linux.com/articles/113936
Here is an example to correct your screen to what should be the right gamma values with the xrandr commands. This is about my dual display install, here the VAG1 display is corrected to the right gamma values:
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root@porteus:~# xrandr -q | grep connected
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 353mm x 198mm
VGA1 connected 1920x1080+1366+0 (normal left inverted right x axis y axis) 477mm x 268mm
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
root@porteus:~# xrandr --output VGA1 --gamma 1.05:1.15:1.136
There is very few well documented hyperlinks on xrandr, here are my best ones, XStrikeForce/HowToRandR12 - Debian Wiki:
http://wiki.debian.org/XStrikeForce/HowToRandR12
The xrandr manual:
http://www.x.org/archive/X11R7.5/doc/ma ... ndr.1.html
And finally, an interesting thread on Grandr, a graphic interface for xrandr.
http://freshmeat.net/projects/grandr_applet/
I hope these links will be useful.
No problem if it is not the case, just come with your questions.

Note: Tell us more about your hardware when you have problems with your display. From:
http://old.slax.org/forum.php?action=vi ... ntID=52631
1) the manufacturer and model of your display could be quite useful.
2) the manufacturer and model of your linux box and of the graphic card. The graphic card spec:
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lspci |grep VGA
lspci |grep Ethernet
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lspci