The title is a line from the song "Don't Fence Me In" written by Cole Porter in 1934 and based on a poem by a Montana state highway engineer Robert Fletcher.
Oliver, i.e. Eddie Albert, and Lisa, i.e. Eva Gabor, own 160 acres and buy 140 acres next to their property, giving them a total of 300 acres--less than half a square mile. By comparison, in 1966 the average farm in the US was 350 acres and had crossed the 300 acres mark in just 1959.