If your replacing just speakers then the doors handle mids only, I would buy a low end but quality speaker to fit in but if your changing the front speakers aswell then I would leave the rear as they are it'll create a better front biased sound stage. just keep in mind the stock amp is weak so don't put anything too powerfull in there, you'll just be wasting money and they'll be underpowered.
As for sizes i'm not 100% but if you search the forum its been discussed before, you'll also need to make some spacers for them from mdf or buy plastic ones.
what are trying to achieve, big bass and clarity or just an improvement over stock. By replacing the front door speakers, dynamat the doors and look into replacing the sub (8"? search forum) in the boot with a better one or you can install a slimline powered sub in there easily will make a good improvement imo. Anything more than that you need to think about adding your own amp and bypassing the stock one altogether but need to be carefull you don't introduce noise as the hu outputs a balanced signal not to mention chopping of wires as no harness is avalable. depends what you want to achieve a few people on here have different setups.