I think the front speakers are 4 inch or 4.5 inch. They screw into the footwell, and protrude into the bonnet area where their back is covered by a black plastic cover. Without changing the rear cover you are very limited to what you can fit as they can't be very deep.
Exactly....they are 4" but anything with a cast metal chassis and a good magnet would need a new cover. Not impossible, but I agree with Alistair, replace the back ones first......
'problem is, (to the best of my knowledge),the S1 did not have any speakers fitted in the rear - only the front.
I would be interested if anyone has summoned up the courage to mount them on the rear alloy 'bulkhead',just above the tank...?
This would necessitate cutting holes in the alloy panel in what is quite an awkward location.
I have an Alpine powered sub fitted up - front and currently have these fitted in the rear -
http://www.richersounds.com/product/standmount-speakers/jbl/control-1/jbl-contorl-1-blk- sound is excellent and they do look good.I have, however decided that they take up too much valuable interior storage so looking at a different solution (see above

)
James