What's the size of a Gravity Generator plate? Since the AoE depends on the size, it's weird the book doesn't say.

Alas, the Gravity Generator tool listed in Into the Storm doesn't help much either with size specifics, but it does say that "gravity generators are huge squares of metal..."

So it's basically up to the GM what 'huge squares of metal' means, but as long as you can pass the Very Hard requisition to get one, I'd imagine that it'd be enough for at least a large room, such as a banquet hall, per unit.

It's debatable on how exactly the quantity rules would come into play, but with a strict interpretation, enough for a single 'huge' room would get the Negligible bonus, while an moderately sized mansion would still get a Minor bonus (it'd cover 10 to 30 rooms) and at Standard difficulty it could conceivably cover a minor estate (50 to 100 rooms).

Of course, if the PCs try to use the plates as traps or otherwise start having them be more useful than simply creating an earth-standard gravitation environment or a fixed prison location, I'd change the scale that can be affected for balance's sake (creating those kinds of traps take more than just gravity plates, etc).

Could you actually place gravity plates on a high ceiling to create a prison zone? If the prisoner goes of the edge he falls to the ground and hopfully dies?

Ph7517 said:

I don't see why not. The description for the Gravity Generators says they are often used as prison cells.

Nothing says that it needs to be in the ceiling, since it can increase gravity, but yes, that's one of the uses specifically listed for the plates.