The crossword solutions for the term 1/640 of a square mile are 'ACRE' (4 letters) and 'SAHARA' (6 letters). An ACRE is a unit of area in the imperial and US customary systems, equal to 4, 840 square yards or 43, 560 square feet. The SAHARA is the world's largest hot desert and the third largest desert overall after Antarctica and the Arctic. It covers most of Northern Africa and is approximately 3.3 million square miles in size. Therefore, 1/640 of a square mile is equivalent to 0.0053125 square miles, which is roughly the size of an ACRE.