Here’s the obligatory xkcd link, discussing practical (and hilarious) metric reference points. Everyone should have this printed out and laminated!
10 square chains = one acre, but 10 square chains != one square furlong. One furlong is ten chains, so an acre is a furlong-chain. (Plough one furrow long, time to turn the team round. Plough a chain’s width of furrows, time to knock off for the day.) It’s a rectangular definition, not a square one. A square furlong is ten acres.
It just happens that a chain is pretty close to 20 metres, so the 1000-acre lot is close to a 2km square.
Thanks!