You may find the following pictures useful. They show the speaker upgrade I've fitted to my car.
I used JBL component speakers (GTO 6507C) - not the greatest quality, but cheap and much more substantial than the standard items (which are unbelievably lightweight!). The cones are also plastic, so you don't need to worry about them getting wet. They are also very efficient (only 2 ohm impedence) so make the most of the factory fit head unit.
I don't think the 6507C is a current model any longer, but it's still possible to find specifications for them on the web and this will tell you all the dimensions you need to know. They
just fit - tight for both diameter and depth - but they will go in without modifying the Multipla door skin and they don't foul the window glass when it winds down.
For the tweeters in the A pillars, I removed the OEM ones:
and made some small brackets out of a strip of aluminium (B&Q sells this sort of stuff) to fit the JBL items to the same mountings:
I brought all the wires back to the crossovers which I fitted inside the bottom lip of the dashboard with some velcro (sorry, no photo for that one).
The worst bit was tracing the wiring for the tweeters. Everything ends up at a multi-wire plug to the right of the fuse tray that's under the drivers side (RHS) glovebox. I cut the relevant wires just before the plug and spliced new lengths of wire to link in both of the crossovers. Again, no photo I'm afraid, but if you're interested I can take a picture and post it here.
It was a bit time consuming but the end result is neat and tidy (it looks no different to the OEM install). The sound is a lot more substantial than the original speakers. I also fitted a pair of JBL speakers in the rear doors. Unfortunately I can't remember the model number for those.