How are the van der Waal's constant 'b' related to the molecular size?

What are van der Waals' forces?

Find out the value of the van der Waals constant 'a' when two moles of a gas confined in a 4 L flask exerts a pressure of 11.0 atm at 300 K. (b= 0.05 L mol^(-1)) .

The unit of van der waal’s constant ‘a’ is :

Write the van der Waal's equation for 'n' moles of a real gas.

Van der Waal's forces are attraction inter molecular forces. Write the names of any two types of van der Waal's forces.

Van dar Waal's equation explains the behaviour of rea gases. What does the van der Waal's constant 'a' indicate?