I've been on Google Earth scoping this area out and doing some comparisons with existing attractions to gauge some idea of what kind of space we have to play with here.
From what I could ascertain from a helpful guide that I saw elsewhere online, the Mouse site alone is around 1,795 square metres, the combined area available in and around both the Alton Mouse and the Alton Beast sites is around 4,005 square metres, and the Adventureland 4-11 space is roughly 2,005 square metres (could be a bit more if you felled the trees separating Adventureland 4-11 and Coaster Corner).
The whole combined area, if everything is amalgamated together and things like trees in the middle aren't considered (I didn't include the trees separating A4-11 and CC in either measurement) is around 7,079 square metres, but as I said, that is in very broad brush terms and literally includes everything within those 3 areas. I did exclude Merlin Magic Making to the North East, the Flag Tower to the North West, and the historic monument to the South, however, so it does take into account some of the boundaries faced.
For some idea, let's look at some existing indoor attractions as points of comparison:
Alton Towers' Other Dark Rides
Alton Towers Dungeon: 2,951 square metres
Duel (excluding garden queue line and Gloomy Wood area): 3,080 square metres
Nemesis Sub-Terra (excluding queue line): 743 square metres
Gangsta Granny (excluding outdoor queue line): 881 square metres
Other Merlin-Built Dark Rides/Areas
Derren Brown's Ghost Train (Thorpe Park): 3,680 square metres
Flight of the Sky Lion (Legoland Windsor): 869 square metres (Sky Lion alone), 4,627 square metres (entirety of Lego Mythica)
Lego Ninjago: The Ride (Legoland Windsor): 1,916 square metres (Lego Ninjago: The Ride alone), 2,843 square metres (entirety of Lego Ninjago World)
Ghostbusters 5D (Heide Park): 1,746 square metres (possibly excluding queue line)
Worldwide Enclosed/Indoor Coasters
The Walking Dead: The Ride (Thorpe Park): 2,563 square metres (excluding queue line)
Van Helsing's Factory (Movie Park Germany): 3,859 square metres
Movie Park Studio Tour (Movie Park Germany): 5,943 square metres
Arthur: The Ride (Europa Park): 6,185 square metres (rough measurement of entire area)
Star Wars Hyperspace Mountain (Disneyland Paris): 6,545 square metres
Revenge of the Mummy (Universal Studios Florida): 9,358 square metres (rough measurement of entire building)
Rock'n'Rollercoaster (Disney's Hollywood Studios): 11,552 square metres (rough measurement of entire area)
Space Mountain (Magic Kingdom): 12,183 square metres (dome alone)
Vogel Rok (Efteling): 3,987 square metres (possibly excluding some queue line)
Crazy Bats (Phantasialand): 5,953 square metres
CanCan Coaster (Europa Park): 1,985 square metres
Worldwide Dark Rides
Symbolica (Efteling): 4,273 square metres (possibly excluding some queue line)
Piraten in Batavia (Europa Park): 4,658 square metres (roughly... knowing the design of that Scandinavia/Holland area, god only knows if this is right...)
Curse of DarKastle (Busch Gardens Williamsburg): 4,280 square metres
The Amazing Adventures of Spider-Man (Universal's Islands of Adventure): 5,732 square metres
Transformers: The Ride (Universal Studios Florida): 3,079 square metres
Valhalla (Blackpool Pleasure Beach): 3,479 square metres
Challenge of Tutankhamon (Walibi Belgium): 2,816 square metres
Antarctica: Empire of the Penguin (SeaWorld Orlando): 4,643 square metres (ride building alone), 7,698 square metres (entire Antarctica realm)
Hotel Transylvania (Motiongate Dubai): 3,672 square metres (including facade)
In essence... I think they have a pretty big area to play with here! Bigger than any other Alton dark ride by a fair margin, and on par with some true dark ride and enclosed coaster heavyweights at other parks!