errata corrige: even if 63 GiB are sufficient for Kickstart, it turns out that current ovirt-engine-appliance-4.4-20200520111649.1.el8.x86_64.rpm needs additionals 197 MiB to install.
At the end, I was able to install oVirt Node with Engine Appliance on top with 64 GiB as local hard drive.