You're right the trim is easy to remove :approve: :
As you can see the front speaker is different to the rears. The outer plastic measures 13cm and the speaker itself is just a bit smaller so I guess it's 12.7cm/5 Inches.
The first 3 visible door fixings are M6 bolts as you say, with a washer too. I'm assuming they are the original ones. They were a bit manky so I took them off and gave them a quick wire brush and coat of black paint whilst I was there. I'm not sure if it matches the original finish but it looks a lot better than before and only took a few minutes:
I didn't see the point in taking a picture of all of them and then uploading them as they are all the same.
Whilst they were off I measured the bolts and found these online which match the specifications exactly:
LINK
I've no idea who the company selling them are or how reputable they are I'm just using this site as an example as it has all the correct details nicely listed.
This is the nut and washer at the bottom of the door by the hinges:
I didn't want to remove this as it was better condition than the other fixings and I didn't want to risk snapping anything off, a 10mm socket fits it though.
Hopefully that is all useful to you too. :approve: