Here's a little helper for the sizes etc for the front and rear of your corsa! These are for the 3 door model but i believe the 5 door to be exactly the same! (Someone correct me if i'm wrong!)
As standard the front door speakers are 17cm (6.5"). When changing the speakers you are going to need a spacer kit to push the speakers away from the door (the magnet's are bigger on aftermarket speakers, resulting in the window only half opening before hitting the magnet!). Again if you decide on 13cm (5.25") speakers you will still need a spacer kit. Halfords sell 13cm spacer kits for around £12.99 (I'll double check price tomorrow) but if you are wanting 17cm spacers these are special order (around £14.99)! I don't know about any other car shops local to you all, they might stock the bigger adaptors! The spacer is screwed into place using the existing screw holes and then the speaker is screwed into this. The correct wiring plug should be supplied with the speakers.
The rear speakers as standard are 13cm (5.25"). For those unlucky like me having the bog standard 1.0ltr you might not have any rear speakers as standard. This again results in a fitting kit being needed. These again can be ordered from halfords (around £12.99) or you could get them from out of a scrap car. If like me you have no speakers then you won't have any wiring either. If this is the case, in the passengers footwell under the side plastic trim you will find two wiring connector blocks. I found that i had my rear speaker wires connected in one side of the connector running from the main loom, but not in the wiring that runs to the back of the car. (The correct location for the speaker wires should be found in a haynes manual as there are loads of wires in these connectors). It's not a difficult job. (Prob best to disconnect battery first before messing with wiring plugs, just to be on the safe side!). Once wires are connected the fiting bracket from what i remember just clips into place! (I will have a look tomorrow and update this if i'm wrong! Has been around 4 years since i did mine
). If you have got speakers already fitted it is a simple case of removing the rear quater panel plastics in boot (torx bit) and then unscrewing the speaker. Again screw your speakers to the fitting bracket and away you go with a much better sounding system.Tweeters can also be found in the front door handle at the top. Prise this panel off (fingers will do) and inside sits the tweeter. To remove it has two pinch points. Fitting an aftermarket tweeter will require cutting the little grille from the covering to enable fitting!
Hope this little guide helps.
I will add pictures when i'm messing with those bits on my car, Which should be soon!
Matt