How to resize Android screen in VirtualBox with UEFI?

I have 64-bit Android 7.1.2, obtained from android-x86_64-7.1-rc1.iso . I installed it on VirtualBox (latest version, 5.1.2.8) with UEFI mode. (The host is 64-bit Linux Lubuntu 16.04.) The installation all works fine apart from the fact that the screen is fixed at 1024×768. The various sizes given in the VirtualBox menu > View > Virtual Screen 1, which range from 640×480 to 1920×1200, are all greyed out. I would like to be able to resize the Android screen, making it both smaller and larger, and even full-screen if possible. Sadly, the usual method, Guest Additions, isn't supported on Android. I've tried following various instructions. The vast majority seem to be valid only for non-UEFI installations, although I tried them anyway. The only one for UEFI that I found doesn't work for me. The methods that I have found include: - Issue the command:
vboxmanage setextradata Android CustomVideoMode1 1152x864x32
(this obviously would work only for that one resolution). - Adding various commands to Grub, either at boot or permanently in EFI/boot/grub/grub.cfg: * set gfxpayload=keep * set gfxpayload=1152x864x32 * vga=ask * vga=834 I am at a total loss how to proceed. Can you help, please?