The best advice I can give is to follow your own feelings. Not every stone, even of the same type, will have the exact same effect. I have basic ways to initially decide on the effect and healing powers of a stone I am considering, but, in the end, it always comes down to what feels right.
Form of the Stone
Basically crystals emit their effects along their edges, with the greatest effect coming from their tip. The form you pick depends on your purpose. An irregular crystal with splintered tips will radiate in all directions. Spheres give off a weaker but a more regulated effect in all directions.
Stones that have been faceted (cut to a shape) seem to be more powerful overall and tumbled stones exhibit more gentle, soft and harmonious energies.
Quality of the Stone
The more closely a stone you are considering resembles a higher quality of characteristics you would expect to see displayed, the better. A clear quartz crystal that appears bright would be better than a cloudy one, a transparent type of ruby would be more powerful than one that is opaque, etc. etc.
But again, follow your feelings. A stone that might look a bit mediocre may have something else in it you may need. Remember, these are only guidelines and not hard fast rules.
Size of the Stone
Common sense would tell us that larger stones would have a stronger effect than smaller ones (power increases with the mass of the stone). A smaller amethyst crystal may only have the effect of a few inches while a large druse could effectively blanket an entire room.
So again, purpose and intent need to be considered… Are you looking to gently heal something or are you requiring dynamite?
It has been my experience that gentle is usually better than a blast to our own personal healing. If you are looking to clear old energies and patterns trapped in a room, then you might consider using an initial big blast followed with smaller stones strategically placed to hold the energy you desire for the space.