I’m just about to upgrade my Bose head unit for either a Pioneer DA230 or Sony AX3005 (and getting Halfords to fit), and I’m wondering if I should upgrade the main door speakers as well?
I seem to recall that there was some issue with sizes, so can anyone offer any advice, recommendations and comment on how easy/difficult it is to do?
TVM! 
Unless you are disconnecting the bose amp I would leave well enough alone. The bose speakers are funny impedances afaik so changing the speakers may end up with some weirdness.
The lower bose speakers fit in a 200mm hole in the door so any 200mm speaker that is not too deep will do the job. I had to use some speaker rings to keep away from the window glass when I did mine
The Pioneer SPH-DA360DAB is about the same price and does Car Play and Android Auto, both wireless. I am seriously considering it myself.
Thanks for this. The reason I was considering the upgrade was because I feel the current Bose set up is pretty quiet, I need to have it up at about 3/4 volume to hear it meaningfully, especially on the motorway. I assume the head unit replacement will rectify this, or have I got it wrong?
You might have it wrong. It depends. It could be the amp getting quieter, in which case a new head unit wont help. Or it could be a fault with the head unit.
Safest course of action if you can afford the speaker replacement too is to replace the head unit and speakers and having new speaker wire directly from the new head unit to the speakers. This is obviously much more involved than a simple head unit swap especially if you want the rears working too. I am not versed in the competence or expertise of Halfords radio fitters so whether or not they are able perform this task is unknown.
As a DIY job it’s not too tricky but it does require removal of the centre console, glove box and door cards to get the wiring where it needs to be