When it comes to water slides, the amount of people a tube can fit isn't very useful. While I'm not one to care about getting grouped with strangers for the large circular raft rides when I'm alone, my wife doesn't really like being grouped with strangers on multi-person raft/tube slides, and I've found most water parks will ask, but don't force it. My guess is the majority of those 5 and 6 person tubes will be going out with at least one spot empty 99% of the time unless they're force grouping. And even I really don't like being grouped with strangers on a tandem seat raft slide, so while 4 person families are the "normal" group size, I'm willing to bet the water coaster rafts will have an empty seat or two more often than not.
I don't have any complaints about VB's capacity, as it looks like it should be at least slightly better than average for a water park, but the size of the tubes probably won't have the impact on capacity you're thinking.
I certainly don't expect every spot on every raft to get filled, but in general I would expect larger rafts to be carrying more people. As a comparison, 5 largest raft sizes of the waterparks in Orlando:
Volcano Bay: 6, 6, 5, 5, 4
Wet 'n Wild: 4, 4, 4, 4, 4
Typhoon Lagoon: 4, 4, 3, 3, 2
Blizzard Beach: 6, 2, 2, 2, 2
Aquatica: 4, 3, 2, 2, 2
As excited as I'll be to see people's reviews/photos on here soon, I'm actually pretty sad that all of the VB speculation and construction photo updates will be gone in 11 days, as is the way of these things! Just F&F to follow for now really, but as most of that will be internal work, it'll be slim pickings unless we count HHN speculation (which I appreciate doesn't appeal to everyone).
OPEN CALL TO UNIVERSAL: BUILD SOME MORE PRETTY STUFF FOR US SOON, PLEASE AND THANK YOU.
The summer will probably be pretty quiet besides Aventura, but I would expect major construction on the first wet n wild hotel to begin late summer or fall, and Nintendo starting late 2017 or early 2018.
Any chance Universal would build a second water park on the 475 acres it owns by the convention center? That should be enough land for two dry parks and a water park, which would essentially match Disney's portfolio.
Given that Universal will probably want to build hotels as well, I would think one large dry park and one water park (or comparable size attraction) with hotels and a CityWalk equivalent is more likely.