Well to solve up the whole discussion on how big it will be, the picture says the dimensions of the boxes.
Forbidden Forest: 191 x 141 x 46 ; Same as Freeing Dobby
The Knight Bus: 282 x 262 x 64 ; Box size in between Quidditch Match and Hagrids Hut
Battle for Hogwarts: 480 x 282 x 61; Same as Hagrids Hut
Now the number of figs may not be dependent on the size of the box, but it does solve the argument on how big the sets will be.
I assume that the Forbidden Forest will be:
A. Scene from Sorcerer's Stone and will include Voldy, Harry, Fang, and Centaur
B. Scene from Chamber of Secrets with Harry, Ron, Fang and spiders (would compliment Hagrids Hut)
C. Scene from Order of the Phoenix with Harry, Luna and Thestrals
D. Scene from Deathly Hallows with Harry, Voldemort and probably one or two Death Eaters.
Since the Knight Bus is bigger than the Quidditch Match, it will probably include Harry, Stan, and maybe Ernie (driver). Hopefully a new Grim too!
Now I think the Battle of Hogwarts will be the courtyard where Harry and Voldemort duel (as seen in the trailer), or the Room of Requirement. It will probably have the same amount of figs as the Burrow. Maybe Harry, Voldemort, Neville, Bellatrix, Hermione and Ron. Probably with a Basilisk Fang, Hufflepuff Cup, Ravenclaw Diadem, Nagini.
But we can't know for sure until we see preliminary pictures.
~MC