PI vs PA Address Space

Why /18? Is there reasoning or research behind this choice?

I'm not sure if there was any analytical justification (not sure how
you'd go about doing that with any sort of reality). I believe it was
a compromise between the people who wanted /14 and /16 and the people
who wanted /19 or /24.

In Danvers I believe the metric "implemented hosts per routing
table entry" was mentioned.

Measured how?