Hmm, I also want to know the population of this country.
select T2.name, T2.population from roller_coaster as T1 join country as T2 on T1.country_id  =  T2.country_id where T1.height  =   ( SELECT max ( height )  FROM roller_coaster ) 
population of the country refers to T2.population; country related to the tallest roller coaster refers to max(height) in roller_coaster and joining with country table on country_id.