How are the van der waal constants 'a' and 'b' related to molecular size? 'a' represents the measure of attractive forces between the gas molecules so how is it related to the molecular size? if a1> a2 then what does it tell us about the molecular sizes of 1 and 2?
The real gas equation or van der Waals’ equation of state is
$$\left(P + \frac{an^2}{V^2}\right)(V - nb) = nRT $$  

where the constants a and b are known as van der Waals’ constants.  

$a$ is a measure of van der waals forces of attraction existing between the molecules of a given gas. Greater the value of $a$ greater is the strength of the forces. It doesn’t affect molecular size.  

On the other hand, $b$ is the incompressible volume per mole for a gas. It is directly proportional to the size of gas molecules.