B6 A4 Speaker Upgrade with Alpine SPR-17LS

Yes, you can fit pretty much any 6.5" speaker in the stock location, but you'll need to make some chunky adapter rings as the OEM speakers stick out from the chassis of the door by about 50mm.
I cut 2 rings from 18mm MDF, about 1cm wide and then a bigger fitting ring to the shape of the OEM speaker from 9mm MDF so I'd be able to screw the whole lot to the door using the original holes.
Glued and screwed the whole lot together. Used a bit of Dynamat round the hole in the door to provide a flat surface. The improvement is tremendous but you really need to add an amp to get the best from the Alpines. The tweeters just slot straight in with a bit of double sided foam tape wrapped round them.

Also, nip down your local Audi dealer and buy a handful of the plastic clippy things that hold the door card on. They are designed so they break when you remove the door card (to prevent damage to the door card), I got through about 3 or 4 per door when I did mine. Luckily I already had a load of them from when I upgraded the interior to leather.
Here's a pic of the OEM speaker so you can see what you're dealing with:

Re: the Dynamat, I just used it to even out the chassis of the door. There a are a few bolt heads sticking out slightly where the MDF will meet the door panel, so I cut a ring of Dynamat stuck it to the opening and then cut away the high spots so that the MDF had a completely sealed flat surface to sit on. Some people go mad with the stuff and cover the whole inside of the car but unless you're going for competition spec then just small amounts in the right place is all you need. You can get a small roll from Halfords for about £15.
For the head unit you'll need to splice both perm live & ignition on from the HU into a perm live. There is no ignition on feed behind the HU, so you either have to switch the HU off manually or have it go off with the engine.
The F240 is actually 4 x 40wrms into 4ohms but that's plenty for cabin speakers.
I found that 5m RCA's weren't quite long enough in my Avant as I'd run power down the drivers side so the RCA's had to go the long way round down the passenger side. I had one set that was 6m which was perfect then had to extend the other set by about 2 feet. The amp was bolted behind the rear right cubbyhole.
One thing to be aware of is that by going aftermarket with an aftermarket amp you'll lose the sub in the boot unless you splice this into the rear RCA's somehow.
I replaced the sub with a JL Audio unit & stealth enclosure so it wasn't an issue.
See my other post for more detail:
B6 stealth sub

Outside diameter was 6.5", inside diameter = whatever the speaker will drop into, so depends on the speaker design. Essentially the ring needs to be about 1" wide all round to give it some strength, but not too big as to foul the door trim. The speaker has to fit inside it and you'll need 2 x 18mm rings per door to give enough depth, + a bigger fitting ring (6 or 9mm MDF) that can be screwed to the door