Hello Matthias,
currently the UEFI sub-systems determines the width of the terminal according to the following rules:
If stdout="vidconsole", use the size of the video console, otherwise use the terminal size from the serial console.
So if stdout="serial,vidconsole" the size of the video is ignored.
With a patch I am about to merge the size of vidconsole will be used if the first 10 characters of stdout are "vidconsole":
* stdout="vidconsole": use video console size * stdout="vidconsole,serial": use video console size * stdout="serial,vidconsole": use serial terminal size * stdout="serial": use serial terminal size
The difference can for instance be seen when using GRUB_TERMINAL=console. This setting allows GRUB to be displayed both on the serial console as well as on the video console.
At least for the Raspberry Pi 400 stdout="vidconsole,serial" seems to be the best choice as it will typically be used with a monitor.
Best regards
Heinrich