What is the name and capacity for the stadium with highest average attendance?
return stadiums ;return attendances of  #1 ;return average of  #2 for each  #1 ;return #1 where #3 is  highest ;return name of  #4 ;return capacity of  #4 ;return #5 ,  #6
['select', 'project', 'group', 'superlative', 'project', 'project', 'union']