How are the van der waal constants 'a' and 'b' related to molecular size?
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.