Every baby is different, so you might have a baby who needs to be changed often/is messy/eats a lot of snacks etc. so size is really dependent on that but I can't imagine you are going to have a child whose things all fit in an XXL cause at the end of the day most babies go poop constantly so diapers will add up, and along with diapers come wipes/creams/etc. That being said there is a reason that majority of diaper bags are made out of nylon/fabric...it's so you can wash them. Kids have a lot of stuff and I am almost certain you don't want to put snacks/wipes/creams (things that can accidentally open and explode) into a Chanel bag. Not to mention the weight of all of the things kids have that can wear down a bag. On top of that, you need them to be able to stretch so you can pack in more stuff if needed. Personally, I suggest against this idea completely. If you really want a Chanel bag, buy a "normal" diaper bag and buy yourself a mini or small flap. Also once you become a mother you will quickly realize of course fashion still has its place but you also need to be more practical about your choices (ex: would you buy a white couch if you have 2 toddlers? probably not).
A Chanel flap of coure does not come with a changing mat because they are not meant to be used as a diaper bag. Personally, I wouldn't go to anyone's home with a child without a changing mat...imagine getting poop on someone else's carpet because you didn't want to take a changing mat because it didn't fit in your luxury handbag. I suggest reading up more about children and what they need on a daily basis before making any decisions regarding this.