Although the thread is quite old, this problem also drove me crazy, may this still can help :
In VirtualBox Manager File -> Preferences -> Display : There isthe Maximum Guest Screen Size parameter : can be automatic/none/hint with
With a size None None or Hint with the desired size ditdid it for me.
FYI : GRUB config : GRUB_CMDLINE_LINUX_DEFAULT="quiet" GRUB_CMDLINE_LINUX="" GRUB_GFXMODE= the one you want (1024x768x32 in my case) GRUB_GFXPAYLOAD_LINUX= keep or the one you want (1024x768x32 in my case) #GRUB_TERMINAL=console (kept in comment)
GRUB_CMDLINE_LINUX_DEFAULT="quiet" GRUB_CMDLINE_LINUX="" GRUB_GFXMODE= the one you want (1024x768x32 in my case) GRUB_GFXPAYLOAD_LINUX= keep or the one you want (1024x768x32 in my case) #GRUB_TERMINAL=console (kept in comment)