I have a grand caravan and it easily fits 6 suitcases-overhead/carry on sizes and a few back packs in the back.
The stow and go under the seat like others have said only fit small things and there is a bar in the middle...let me know if you want pictures...just ignore the dirt, it’s winter and I won’t vacuum it yet!
That being said, we have rented one as well and squeezed 7 of us in it. It was DH driving, myself either in the back row with 2 of my kids (under 14 at the time), my sister, mother and my other kid. We would just take it to the beach and it fit all 7 beach chairs, a cooler, a few beach bags, boogie boards, fishing poles...but to drive any length of time in it, I would not. So the few times I got stuck in the back row with my kids it was not comfortable. I felt a little cramped so if there were 3 adults in the back, no one should be over 130 lbs, IMO. Seriously in the back row, my oldest was probably 120 lbs, my son was probably about the same and me who wishes I was that size, but even if I was it was too cozy for my liking and they are my kids, lol!
Last year we were fortunate to get upgraded to a full size GMC Yukon XL that seats 8. I sat in the back for a few rides when we had my BIL with us and it had more room then the van. It was more comfortable too and made our drive to the Keys oh so enjoyable! So much in fact I told my DH we were renting one again this year and he said no because he figured a few thousand but shockingly it is *only* $950 for 10 days. Oh and the space in the back, we didn’t have to play some sort of game trying to squeeze all our beach crap in there.