According to the Sunflower Books Alton Towers Guide, the Planetarium dome was 20-foot diameter. I remember going in it, but it was about as memorable as erm, whatnot.
According to the same guide, here are some facts about the Model Railway...
Two men took two and a half years to build it, working an average 16 hours a day.
The landscape contained two tons of plaster.
Over 100 gallons of water circulated through its rivers and lakes.
72,000 track spikes held 1500 feet of track in place.
Over 10 miles of wiring was used in the electrical circuits.
200 relays controlled the movement of the rolling stock.
There were over 3000 trees, 400 lights, 70 colour signals.
35 locomotives pulled 200-plus items of rolling stock (manufacturers: Wrenn, Liliput, Fleischmann).