Passive RFID tag - link between size of antenna and read range (detailed)?
The answer lies in Friis' Equation that describes that relationship between power density and reception.  In passively-powered RFID tags, the charge pumps are the limiting factor as you need enough voltage to get them started.  This is why there's a balance between antenna size and power required to get the charge pumps started.

If you want to dive into the circuits and equations, you should ask google for my RFID 2018 slides (which currently seems to be down); however, the tutorial from 2016 is here.

Regarding the specifics of the read range, because the interrogator provides a coherent clock, you have a read range that is effectively unlimited if you give the tag unlimited power.  The caveat is that you need enough power at the tag to power it up.