A 20GB dom0 usually fits all your needs. For installing Kali, you only need temporary space - again, no use for it after installation.
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clean up dom0 to make it pass through
How to Cleaning Up Disk Space in Dom0? (in terminal) - #5 by padhi -
increase the size of dom0 (typed in dom0 konsole: )
sudo lvresize --size 40G /dev/qubes_dom0/root
sudo resize2fs /dev/mapper/qubes_dom0-root