The horde rules in Deathwatch appear to be how the game intends to represent large, tightly clustered mobs of enemies.
How do I represent hordes on the table? I use 1m squares. Do I move it as a solid mass, or a fluid mass? (Example: Can it or can it not flow through a narrow alley?)
The size is determined by how high the magnitude is, which I believe the book details how much total space that takes up in the game rules section.