I wouldn't recommend too many hard containers either because as a PP said, you just can't smash them in like soft sided things and they're not easy to carry either. I don't think same sized suitcases matter too much because if everyone has to deal with a suitcase it makes fewer people to carry other stuff. We took 3 suitcases for 4 of us. We have a very large Jeep brand suitcase, another not quite as large suitcase, and a smaller one which is a little bigger than carryon size.
DD10 packed in the small one with room to spare. She is picky so I packed each of her outfits in gallon sized ziploc baggies. It made getting ready so much easier and everything seemed to fit in the suitcase better. I also packed extra shoes in our suitcases in gallon sized baggies.
I packed my stuff in the largest one. DS5 packed his stuff in the next largest one and DH's cargo shorts take up so much room he split his stuff between mine & DS's.
We planned to stop on the way there and took an extra suitcase with clothes for each of us in it.
We always pack toiletries, my makeup, medicine, etc, in another small shoulder bag.
For food, we took a large cooler and put it in the floor of the van in the middle between the front and 2nd row of seats. We took a large soft sided cooler with lots of compartments for snacks and boxes of cereal and kept it in the back seat. We used that to bring souvenirs and any other extra stuff back in it.
Other miscellaneous things:
We took a beach bag full of beach towels and a couple of sand buckets because we stopped at Daytona.
We also took a very small back pack into the parks. I had baggies for each day with different things we needed like ADR confirmation #s but that seemed to be sort of overkill. I also had a box packed with those baggies surprises for the kids to keep them from getting bored during the drive, and stuff to decorate our room.
The kids each took a back pack with toys, DSIs and other stuff they chose to take.
We have a compartment in the back of the van to keep oil, a lug wrench, jack, etc. I was worried about something happening and that we would need to unpack the van to reach any supplies we might need for the vehicle. So I packed a battery powered tire pump, quarts of oil, a good lug wrench & jack, etc.
We had plenty of room for our stuff and we were gone 11 nights.