the front speakers are 13cm as for the tweeters i have yet to look at them, im pretty sure the rear seat speakers are 13cm too. if you change the headunit, you dont lose the dash display, as soon as you unplug the standard headunit, you lose the radio info, but the gap gets filled with the date instead, everything else works fine! as for the steering controls if you buy any sony unit, the lead is £32.99 from halfords (PC9-112) you can also get them for some alpines, kenwoods, pioneers now aswell, but im cant recall the prices. pop into your local halfords and get them to check for you!
Justin