i presume it was too big? you will probably need a 10mm tube which is like the 'middle' size
i hate the way spark plug sizes go, here is something that may help though
8mm (Euro 14mm)
10mm (Euro 16mm) - this is what you will need
14mm (Euro 21mm) - this is probably what you bought
Would it be the same size socket required for the Focus mk1?