With Alamo you get your choice of the Chevy Venture, or the Dodge. They have some Grand Caravans, and some regular Caravans. All of the Ventures we have had have been the extended length. Caution - I own both a Chrysler and a Venture, my Chrysler, like the Caravan, allows you to slide forward the rear seat, most Venture's do not, but some of the Ventures have a split folding rear seat that might help with the luggage.
We rented the larger Astro from Alamo on our March trip. Don't really think it had anymore room in back than my Venture. We were able to get 5 adults and three mid sized children, 4 suitcases, and 2 sets of golf clubs in the van. If you get creative where you put things, under and between seats, etc. you'll be amazed what you can cram in any size van. The ride from MCO to WDW isn't that long, so you should be able to survive.
I've got a mini-van reserved for Aug from Alamo. Depending on how much luggage we take for this trip, I'll "shop" the vans to find the one best suited for our needs. We've done this the last two times, and have actually started out loading one van, then switched to another.
Didn't have any trouble with renting from Alamo except that they included coverage I didn't want, and ended up having to go to the Alamo site on WDW property to have it cancelled. Be sure you carefully check what they add or don't add to your contract BEFORE you leave the counter.
