Does height matter when using the scalar field? Can the boxes be on a table, or must they be on the floor if I'm lying down?

Height is not critical when using the scalar field.

The Scalar field fills the space between the Transmitter and Receiver, so the important thing is that your body is within that field. The boxes can be placed on a table, on stands, or on the floor—whichever is most convenient for your setup.

If you're lying down, many users place the boxes on tables or chairs at either side of the bed. As long as your body is positioned within the scalar field between the two units, the exact height is generally not important.