When does a real gas behave like an ideal gas?

The compressibility factor $(Z)$ of a gas is greater than unity at $1 \, atm$ and $273 \, K$. Therefore:

"Real gases do not follow Boyle's Law,Charles's Law,and Avogadro's Law perfectly under all conditions." Explain.

At low pressure,the Van der Waals equation is reduced to:

For real gases,the relation between $p$,$V$,and $T$ is given by the van der Waals equation:
$(p + \frac{an^2}{V^2})(V - nb) = nRT$
Where $a$ and $b$ are van der Waals constants,$nb$ is approximately equal to the total volume of the molecules of a gas,and $a$ is the measure of the magnitude of intermolecular attraction.
$(i)$ Arrange the following gases in the increasing order of $b$. Give reason: $O_2, CO_2, H_2, He$
$(ii)$ Arrange the following gases in the decreasing order of magnitude of $a$. Give reason: $CH_4, O_2, H_2$

The compressibility factor for a real gas at high pressure is $:$