The base ostree ref is available from the same Kinoite repo. The code is in the -variants branches in my workstation-ostree-config fork and in PR waiting to be merged upstream.
It is based on the packages common to all variants but I have no idea how much space it takes. I created it for those that want to use a minimal desktop environment such as Sway that can easily be overlayed on the base image. If you need anything more server oriented, Fedora CoreOS is a better (official and properly maintained) alternative.