|Cool, thanks guys. Just needed to do as suggested by slh:
you can add "vga=791" (or "vga=794") to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub and run update-grub afterwards.
No need for gfxpayload entry which is one I'd seen suggested while searching previously.
For the sake of completeness, here is the relevant excerpt from the wiki devil linked to, apparently this is the new way to do it:
* Uncomment (remove '#' from beginning of the line) to change (increase) resolution of your boot loader. Use one of classic (4:3 rate):
o GRUB_GFXMODE=640x480 (is used if isn't here a proper value)
or make experiments with some others nonstandards like widescreen (16:10 ratio in my case), here is list of most of commonly use Display resolution:
* If you don't know what modes are supported by your graphics card, go to the grub command line and run 'vbeinfo'. It will list all available modes. (You might need to run 'insmod vbe' first if the vbe module isn't loaded yet)
* Use this setting to set the framebuffer resolution (replaces the vga= kernel option). It uses the GFXMODE values with additional color depth. Use 'vbeinfo' from the grub command line to list what modes are available to you.