Is there a way to make the Mac OS X guest OS adapt the resolution of the host OS given that there's no guest additions yet as of the moment?
That should help solving your problem.
Note: Install Multibeast 4.6.1 first (registration required)
There is no way for it to automatically adjust, but you can specify a custom resolution for the guest to use.
You can manually edit the VirtualBox configuration, and the boot file in OS X to use a custom resolution. It's a two part (Host VM/Guest) process to enable the resolution.
Here are the steps: