The front speakers are 16.5 cm diameter. For some reason I've never been able to work out any speakers labelled at 6.5", 16.5 cm or 17cm will fit.
See the ICE FAQ
for a link on how to change them.
The front speakers link to the tweeters as well, there is also a how to change the tweeters in the ICE FAQ
It's best to change them with a component set of speakers, that is a 16.5 cm mid / bass unit, a cross over and a tweeter for each side.
Cheap speakers (£20-£40) aren't really worth the cash, but once you get to £50 the sound quality really does improve significantly compared to the original units.