For any future people with this problem, the best documentation that I know of is https://www.kernel.org/doc/Documentation/fb/modedb.txt although it’s not exactly newbie friendly. My understanding is that if you are using the open source KMS drivers (e.g. intel, nouveau, radeon) then you can set the video mode on the kernel command line.
Graphics stuff is just very confusing and in the past I have (regrettably now) thrown a computer away because I didn’t have the patience/info to figure it out. Hence, my interest in improving things!
A DVI-I connector (analog and digital on the same connector) just complicates things further, but any VGA adapter should be fairly simple and standard (cough). How it is wired in the computer though…?
It might just be a case of needing to enable VGA, which would be video=VGA-1:e
To list available connectors, at a terminal use
ls /sys/class/drm/
and if you want to be really fancy (from https://wiki.archlinux.org/index.php/kernel_mode_setting)
for p in /sys/class/drm//status; do con=${p%/status}; echo -n "${con#/card?-}: "; cat $p; done
Since you are connected into a DVI-I connector, you probably should be using something like
video=DVI-I-1:1280x1024M@60e