Crystals work in a fairly obvious way. As they move, light glints off of their facets, and they can make rainbows. The more attention-getting, the more effective. Large is often best, but feel free to use any size crystal that looks appropriate for the size of the room. The crystal should not be out of scale—neither too large nor too small. Even more important than size is shape.
There are two shapes of crystals that are commonly used in feng shui. The round disco-ball shape is recommended for circumstances where people will be passing under it. The roundedness of the shape is completely friendly. It is quite comfortable to hold in your hand.
However, if sunlight reaches the crystal (in a window or skylight), you may want to use the kind that is octagon-shaped. They are unsurpassed for making large brilliant colors. The best octagon crystals have only a few facets (cuts), which are fairly large. I’ve noticed some octagon crystals (rather recently on the market) that are quite inferior. They have many small facets radiating from the center like a sunburst, and do a very poor job of making colors over a large distance. (See the drawings at the bottom of this post.)
Another crystal shape is good for creating rainbows—it’s called dreidel, named after a Jewish spinning top. Because the bottom is pointy, it should only be used in windows where it cannot aim directly down onto a person. The dreidel shape is therefore not appropriate for use in skylights.
There is one shape of crystal that is never appropriate in the home—that’s the teardrop or icicle. (An exception can be made for a temporary holiday ornament.) The shape is too downward, and that’s not a good direction to aim chi energy. Teardrop crystals are too sharply pointed and therefore create poison arrows. Some crystal chandeliers have tons of teardrop crystals and, no, they’re not ever recommended in feng shui.