Without VirtualBox guest additions installed, the only screen resolution is 640x480:
$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480
default connected primary 640x480+0+0 0mm x 0mm
640x480 73.00*
How can the resolution be set to 1280x1024, for example? No VirtualBox guest additions should be necessary.
The guest is a Fedora system.
I've tried adding an Xorg config file as described by this Whonix article, which did exactly nothing.
Using cvt to get a new modeline for xrandr does not seem to work either:
$ cvt 1280 1024
# 1280x1024 59.89 Hz (CVT 1.31M4) hsync: 63.67 kHz; pclk: 109.00 MHz
Modeline "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
$ xrandr --newmode "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync
xrandr: Failed to get size of gamma for output default
$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480
default connected primary 640x480+0+0 0mm x 0mm
640x480 73.00*
1280x1024_60.00 (0x1bd) 109.000MHz
h: width 1280 start 1368 end 1496 total 1712 skew 0 clock 63.67KHz
v: height 1024 start 1027 end 1034 total 1063 clock 59.89Hz
$ xrandr --addmode default 1280x1024_60.00
xrandr: Failed to get size of gamma for output default
$ xrandr --output default --mode 1280x1024_60.00
xrandr: Failed to get size of gamma for output default
xrandr: Configure crtc 0 failed