I'd like to cool a Raspberry Pi 4 Modell B with passive cooling only. To get the heat away from the chip I'd like to use a heat pipe and radiator to distribute the heat. Something like this. My problem is, that I don't know how to dimension the heat pipe + radiator properly for RPi4 not overclocked and run under industrial conditions, means a temperature range between -40 to +85 °C. Does someone have experience and/or a recommendation?
You definitely should not buy from the vendor you've linked to - this.
Why? Because they provide no spec sheet.
Repeat the following mantra 3 times each morning when you wake up:
NO SPEC SHEET - NO SALE!
You need thermal resistance and airflow curves which are typically derived empirically by the manufacturer. You could do this yourself if you had a laboratory and proper instrumentation, but if you had that equipment I doubt you'd be asking this question here.
This is crap - do not buy it. If you're still interested in heat pipes as a solution, you might peruse this answer for some more details & ideas.