I’ve conducted tests with overlayfs and published the results here Whonix in live mode in Qubes OS! Amnesia and anti-forensics mode - #20 by bigboy.
In this comment, I’ll just add the test results with size=100%: you’ll feel a very strong memory load when working with virtual machines in live mode. However, size=100% had no effect on it. I tested memory load in Qubes with size=100%, 90%, 80%, and 70% - it made no difference. The memory consumption remained the same, and I launched the same number of virtual machines each time. The only thing that changed was the free disk space
So, size=100% is the most efficient for working in live mode