Question is, how detailed (polycount) should such a small object be?
Shadowcasting polys matter a lot more than the actual surface mesh, though for something the size of a chess piece, you'd probably want to stay under 100 polys for each piece, with a simple 10-20 poly shadowmesh.
How does the scale in Blender translate to the engine?
Don't know, but you could always import an existing model for comparison.
Could the pieces be scaled up or down in the game?
They can, but it's a bit hacky. If you want to make larger versions (they might make good statues or decorative pieces) I'd model them separately.
Could placing a specific piece on a pressure plate make something happen?
Yes, if a mapper set it up that way.