First check if your computer knows about this resolution by typing this into a terminal: xrandr
You will get an output more or less like this:
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
DisplayPort-2 disconnected (normal left inverted right x axis y axis)
HDMI-A-0 disconnected (normal left inverted right x axis y axis)
DVI-D-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 521mm x 293mm
1920x1080 60.00*+ 50.00 59.94
1600x1200 60.00
1680x1050 59.88
1280x1024 75.02 60.02
1440x900 74.98 59.90
1280x960 60.00
1280x800 59.91
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
720x576 50.00
720x480 60.00 59.94
640x480 75.00 72.81 66.67 60.00 59.94
720x400 70.08
and the line: 1280x1024 60.02 (or something like that) should be in the list.
If it is not in the list, the system doesn't offer you the resolution you need.
The most common reason is that the monitor's EDID is wrong (in other words, your monitor is lying to your computer)
The best thing to do is add the resolution parameters yourself so the system can use it.
Follow the instructions of the first answer here (just replace the values in the example for your own values) and also read the comments:
So to summarize the above link, theoretically the following terminalcommand should fix the problem permanently:
cat<<SCREENFIX > "$HOME/.xprofile"
#!/bin/sh
xrandr --newmode "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
xrandr --addmode VGA-0 "1280x1024_60.00"
SCREENFIX
Afterwards, logout and login again to activate this fix.
(and use xrandr to check if it worked)
But please read the linked article first.
good luck 