A closed loop is a sealed box full of water that you then heat. Three things follow, and each has its own relation.

First, the water grows. $\Delta V = V_0 \beta \Delta T$ — delta-V equals V-nought beta delta-T. $\Delta V$ is the expansion volume in litres, $V_0$ — said V-nought — is the cold system volume in litres, $\Delta T$ is the temperature rise from fill to operating in kelvins, and $\beta$ — the Greek letter beta — is the volumetric expansion coefficient, about $4.6 \times 10^{-4}$ per kelvin over a heating loop's range. It carries “per kelvin” and nothing else, so the litres in and the litres out are the same litres.

Second, something has to receive that growth, or the relief valve does it for you. $V_t = \dfrac{V_s e}{1 - P_1/P_2}$: $V_t$ is the tank volume, $V_s$ the system water volume, $e$ the net expansion as a fraction of that volume, and — this is the part people get wrong — $P_1$ and $P_2$ are the cold fill and maximum pressures, both ABSOLUTE, meaning gauge plus about 101 kPa. The subscript convention is simply low then high: 1 is where the loop starts, 2 is where it is not allowed to go. Feed gauge pressures into that denominator and the answer is quietly, confidently wrong. The denominator is the pressure band you are allowed to use, and a narrow band always demands a bigger tank.

Third, the water has to reach the top of the building and stay above atmospheric when it gets there. $P = \rho_w g H + P_m$, where $H$ is the height from the fill valve to the highest emitter in metres and $P_m$ is the safety margin — typically 20 to 30 kPa. Because $\rho_w g$ is about $9.79\ \mathrm{kPa}$ per metre, one storey of about three metres costs roughly 30 kPa, and a ten-storey riser needs nearly 300 kPa before you have added anything at all. Skip the margin and the top-floor vent sits at exactly zero gauge, which is where an automatic air vent stops letting air out and starts letting it in.