Welcome to the forum, Fraser!
Doing a search for 'speakers' in the Audi discussion section (including archived items, so check that option or you will find too little!) will reveal enough info to give you the basic idea. For example:
http://www.a2forum.net/forum/topic.asp?TOPIC_ID=1742
But the basic info is:
- front and rear speakers (the bass units) are about 16cm in diameter, BUT are custom-made out of one piece (adapter and speaker are one)
- adapters, specifically for the A2 do not exist
- making your own adapter is a bit of a chore, especially for the front, where you need to take the doorn catch into consideration (read the posts you find with a search. it will be clear then)
And many people are very happy with the Bose systems that Audi itself offers.
And other are just very good at DIY and go 'all the way'
http://www.a2forum.net/forum/topic.asp?TOPIC_ID=1749
If there's anything else you'd like to know or if you couldn't find something, feel free to ask again, of course.
--
Edo
(2003 Silver 1.4 Tdi Exclusive(SE), black interior, sport seats, heated mirrors, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)