All kinds of gas have (roughly) the same number of particles per unit volume, assuming constant temperature and pressure. The separation between particles is enormous compared to size of the particles themselves, so the size of any gas particle should be considered negligible. The type of gas therefore has very little effect on the volume it occupies.
The number of particles in a gas is given by n=(PV)/(RT), where:
n = number of moles of particles (1 mole = roughly 6.02×10^23, I think)
P = pressure, pascals (N m^-2, if you want to get technical)
V = volume, m^3
R = Molar gas constant (8.31, JK^-1mol^-1)
T = Temperature (K)
I just helped you learn physics. You're welcome.
/completely off-topic physics
@Platinum
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