It really doesn’t. The in-qube filesystem on the ‘private’ volume (ext4 by default) is not particularly related to your question of how large that volume can be (at least not in the low terabyte range) nor to my response about dom0 storage layouts.
Even if someone wanted to use a different filesystem like Btrfs inside the ‘private’ volume for whatever reason, there’s no need to “download Kaisen Linux” or anything like that. The simple way would be to run e.g. mkfs.btrfs
in dom0 on the uninitialized volume, i.e. before the first start of a newly created qube or after clearing it.