They're 4" with 1" tweeters.
They are a straight swap, but you'll need to use adapter rings or make your own from MDF as the OEM units use a 3 point fixing that sticks out from the speaker surround.
You can either fit 4" components or go up to 5.25", but the depth is quite tight (only 60mm) so you'll need slim fit 5.25" units.
Really depends how much you want to spend, I'd recommend £120 max.
Alpine Kenwood or Pioneer for around £60-70, or Infinity Kappa's 50.5 for about £120.
The downside of the Infinity's is that they're a pain to fit as they use prope crossovers which are really too bulky to fit in the doors so you may need to rewire them and mount the Xovers behind the glovebox. I'd recomend the cheaper units with inline Xovers, as you're driving the fronts off the puny amp in the HU, so you won't get the benefit of more expensive speakers unless you're adding an amp. Even the cheapest replacements will be a massive improvemen on the paper cone crap that Audi seem to think is all a £20k+ car deserves.
