Hi,
The sizes of spark plug "sockets" are 10mm(16mm) or 14mm(21mm) for cars and 12mm(18mm) for motorbikes.
I say "sockets" because I think thats what you should use, you don't want to cross the thread when refitting with a spanner, thats a head off rework job.
Not sure about the sporting but the 899cc uses the 14mm(21mm) size socket - plug size(actual size)
Thanks
Paul T