The speakers are 13cm (5.25").
I put the Pioneer TS-Q131C's in the font of mine, which are custom made for Renault, although any 13cm speakers should fit ok. The standard speakers in mine didn't have a tweeter under the covers in the dash, just a blank under the covers, but the wires were there for the tweeters so I just wired them straight in. Take care if removing tweeter covers as they can break when you take them out and the covers don't quite fit right with the new Pioneer tweeters, although only a slight gap, which I'm able to live with.
Sound is improved over standard but still not brilliant.
Eventually I'd like an amp plus some 13cm woofers in place of the back speakers, which I think would give a reasonable sound without taking up boot space.