This is the specification that decides everything, and it is the one nobody is ever asked for.

What a facet actually does

Each flat mirror throws one beam. More facets means more beams, each smaller and dimmer; fewer facets means fewer beams, each larger and brighter.

That trade sounds neutral until you add distance. A small beam loses definition as it travels, so in a big room a fine-faceted ball dissolves into an undifferentiated shimmer long before the light reaches the far wall.

Match it to the ceiling

Low ceiling, small room: 10mm. The throw is short, so fine beams stay crisp, and the density fills a space that a coarse ball would leave patchy.

High ceiling, large room: 25mm or 50mm. Fewer, harder beams that survive the distance and still read as separate points of light on the far side.

Diameter is a separate question

Diameter controls how much light the ball can collect and therefore how bright the whole effect is. Facet size controls what that light looks like when it arrives. They are independent, and a big ball with the wrong facet size is just a brighter version of the wrong effect.

The test that costs nothing

Stand where your audience will stand and look at the far wall. If you cannot resolve individual moving beams there, the facets are too small for the room. If you can count them on one hand, they are too large.