How big is an acre?

As wildfires rage across Southern California, news media regularly report the sizes of the largest and most damaging fires in the region using acres as the unit of measurement.  But exactly how big is an acre?  While listening to the ongoing coverage of this current wave of fires, we at Geospatial @ UCLA became curious about how well people understand units of area -- specifically, to what accuracy can people who are constantly exposed to land areas expressed in acres actually identify an acre of land?