For those of us that regularly plow large tracts of land using manual tools, this is an extremely useful unit. Anyways, if that isn't a usual activity for you, an acre is an area of 10 square chains, or roughly an area 1 mile long and 8 feet wide.
Nah you'd either overestimate because you're not starving and disfigured from the knights from the neighbouring manor maiming you for fun, or you'd underestimate because no one is whipping you and threatening to kill your children if you don't meet your quota.