If command xrandr gives something like
$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1024 x 768, maximum 1024 x 768
default connected 1024x768+0+0 0mm x 0mm
1024x768 0.00*
800x600 0.00
640x480 0.00
Resolution can change using a command
xrandr -s 800x600
If wanting resolution 800×600
Desired resolution can add to the startup file.
geany $HOME/.desktop-session/startup
Settings can be saved to the $HOME/.screenlayout/default.sh file also. An example:
#!/bin/sh
xrandr --output default --mode 1024x768 --pos 0x0 --rotate normal
With proper /etc/X11/xorg.conf file, resolution can reach 1280×768 instead of 1024×768
Computer is an old Fujitsu and graphic card is notorious Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx
Read also: https://puolanka.info/goto/how-to-make-a-xorg-conf-file/