Well I finally figured out how to fix the X.org issue with running Fedora 23 under VirtualBox, where you can't install the vbox driver for video. This limits the VM to a max of 1024x768. The fix is to downgrade the x.org drivers to F22's version.

sudo dnf --showduplicates --allowerasing --releasever=22 downgrade xorg-x11-server-Xorg

This will roll the packages back. Note, after you do this, dnf/yum upgrade will reinstall the F23 packages. To prevent this, edit /etc/dnf/dnf.conf and add the following line at the end\

exclude=xorg-x11-*

Then reinstall the VirtualBox drivers as usual, and you should get your normal options for screen size back. Eventually there will be an update for VirtualBox to fix this, but until then we've got this.