If you are experiencing what I am, this appears to be working...
In the old days, to flash a Nexus Device, wugfresh's Nexus Root Toolkit was what I used. With my current Nexus 6P, I'd recently been tinkering with a newer LineageOS unofficial ROM, and somewhere along the way, I ended up with that "Couldn't parse partition size '0x'" error, and sometimes others, when trying to flash the original ROM back, and was afraid my phone was history. However, I remembered wugfresh's Nexus Root Toolkit, and that it had options in it to "unbrick/debrick" a nexus device. So, I took my already downloaded latest official GOogle Nexus ROM, hunted down the wugfresh nexus root toolkit, and then it was readily able to restore my phone to its previously functioning state, just like in the old days. Now, at this very moment, I just did this... I don't know if anything else will pop up not right, but it didn't give any errors when it went to flash the official rom, it rebooted, and is now waiting for me to set up my nexus phone... lol... so it looks as though the crisis has been averted... Since it is no longer supported, and bitdefender throws up warnings when i try to go to their original website. I used the techspot's website to download the toolkit. And, of course, the official google android rom site for the latest official ROM.
1. Downloaded Wugfresh Nexus Root Toolkit.
2. Downloaded latest Nexus 6P (angler) official ROM.
3. Installed Wugfresh Nexus Root Toolkit.
4. Put phone in Bootloader and connected to computer.
5. Ran Wugfresh Nexus Root Toolkit
6. Manually selected Nexus 6P.
7. Selected any ROM version.
8. Under (Restore/Upgrade/Downgrade) - Flash Stock + Unroot ... I selected "Soft-Bricked/Bootloop ... and then hit "Flash Stock + Unroot"
9. Selected "Other/Browse... For use with any factory image package: zip/tgz"
10. Selected "I downloaded a factory image myself that I would like to use instead."
11. Clicked OK.
12. Selected the previously downloaded latest Nexus 6P (angler) official ROM.
13. Watched with gratitude as it seemed to resuscitate my favorite phone.
I hope this helps someone else who may have fallen into this... I'll post again if it turns out not to have solved the problem fully.