How big is an air tank of compressed air at 3600psi if the compressed air should hold 600MJoule?

Question

or maybe even a link or formula?

Answer 1

Work = integral of PdV.

Integrating, W = PV from v1 to v2.

W = 600MJ

Pressure = 24.82kPa

V = W/P = 24m^3

Answer 2

First, convert your pressure from psi to Pascals.  Then, the stored energy in the tank is just pressure times volume.  You already know the energy and you will have pressure in the correct units (1 Pascal = 1 N/m^2).  Then all that's left is a little algebra.