An acre is 1/640 of a sq mile, or for those using the decimal metric system, 4.046,8 sq m. It is a concession to my English speaking readers to use such a unit, so convenient to them.

As not to force to much the concession, my first examples are in Paris: an empty acre (place Dapuhine, see on http://binged.it/12AK5LG) and a full one (La Madeleine, http://binged.it/WF7MNJ). So, on an acre you can have a charming small square or an imposing neoclassical temple.