What are the names of the countries and average invoice size of the top countries by size?
return countries  ;return invoice sizes of #1 ;return the  average of #2 for each #1 ;return the  highest of #3 ;return #1 where #3 is equal to #4 ;return names of #5 ;return #6 ,  #4
['select', 'project', 'group', 'aggregate', 'comparative', 'project', 'union']