There are currently 299 GiB remain with my computer, but I cannot share it with the installment Qubes. Naturally, deleting it is not an option. For some reasons, there is 55 MiB available to be use with Qubes, but that is far too small.
How should I allocate or split the remaining 299 GiB to Qubes (if that is the right action to take)?
I’m not sure if this is an option for you but I’ll just put it out there in case. If you are talking about a desktop system with an empty slot you could add a sata controller and a sata docking station. (USB could work too)
I have one where the sata controller always tries to boot off of a drive in the docking station and failing that the internal controller boots from the internal drive. So, If you installed qubes on the external drive it would boot from that but failing to find the docking station powered up it would boot your windows system normally.
Security wise your qubes drive could grow wings (or be locked in your safe) and the Windows OS would never even see the qubes drive, so it could not modify it.
The trick is getting the boot sequence to load from the ext sata controller first. It takes just a few seconds to realize there is no drive so that it boots normally to your Windows.