virtualbox.org

The user manual ch. "3.14.1 Video modes in EFI" needs to be updated to include the new options for the EFI display settings. Both the added resolutions as well as the new commands for their setting:

• 
  VBoxManage setextradata "<VM name>" VBoxInternal2/EfiHorizontalResolution value="xxx"
  VBoxManage setextradata "<VM name>" VBoxInternal2/EfiVerticalResolution value="yyy"
  

And I can't seem to find the available resolutions in the source code at the moment...

OKOK... in 5.2.0_RC1 the situation is still is as you mentioned. The code is in a half-finished state and therefore the manual isn't up to date. This state will be forgotten rather soon, which is why I'm replying here to make sure that people can find out about the final state in 5.2.0 (no plans to backport this to 5.1.x).

With the 5.2.0 release the preferred way to select the resolution will be

VBoxManage setextradata "<VM name>" VBoxInternal2/EfiGraphicsResolution HxV

With H being the horizontal resolution and V being the vertical one. The reason is that setting 2 extradata entries to select the resolution is annoying (and is an unnecessary invitation to typos and inconsistencies). The separate config will also work if the short one isn't provided (but your syntax with value= was never accepted!). You can pick from 31 standard monitor resolution definitions listed in the manual, and if that's not enough you can define your own resolutions (fully reusing the VESA custom mode support, even though EFI doesn't have any VESA BIOS or VBE as such, but I saw no gain in inventing something new).

5.2.0 release will have this all sorted, in fact the latest trunk test build has everything in place (including docs).