Well I can't guess at your rear speaker size, but if you've got 5.25" in the front, you'll likely have 5.25" in the back. (I can take a look at your car if you want).
I have an E46 as well, but mine was made for the US market and uses 6.5" all around. - I think its best to just look at your car.
Now, as for the sound... MB Quarts are pretty damn nice. This setup will knock your socks off when it comes to mids and highs. (I wont lie, no 5.25" speaker can give u bass like a sub can).
In my honest opinion you really should amp them. A simple 2 channel amp will do the trick - run both your lefts to one channel, and both your right speakers to the other. Unless you want to watch 5.1 Dolby digital movies, you don't need surround sound.
If you dont have the $$ for an 1) amp, 2) amp witing kit, and 3) amp installation - you can run them off of your factory deck now.. and grow into an amp later. MB Quarts are a pretty high end speaker, and they really need an amp to drive them at their optimal sound output.
Now if you never want an amp setup and are happy with the power coming out of your deck now... then I'll sadly suggest you dont buy these - go for a modestly priced pioneer/jvc/whatever <-- While they will not deliver acoustic fidelity, and build quality, these lower end speakers are made to be driven at low input power.
I hope I've answered your questions.
PS. I can install whatever you want (however you want it) when it comes to audio... but we can talk about this later.
PPS. hmm, My E46 machine has a factory amp... does yours?