Noble gases have larger atomic size than the preceeding halogens because

• The size of the atom of an element decreases as we move from left to right of a period due to the increase in Ionisation potential and effective nuclear charge. Now the trend continues till halogens. 
• The noble gases have full filled outer and inner shells so there comes a repulsive force between the electrons resulting in a larger atomic size compared to the halogens. 

Therefore, Noble gases have larger atomic size than the preceding halogens because Vander Waals radius is considered in them.