Somewhat of an oxymoron - speaker stands for the floor for bookshelf speakers.

How much the sound varies with exact placement varies highly with the speakers. Ribbon and electrostatic are very sensitive to placement… as in 1/8th inch can make a significant difference. While this is a topic a different thread.. I think it is often possible to get great sound from ribbon speakers with a much lower percentage investment in the speakers… typical rule of thumb is 30%… for many years I had 10% in speakers (Apogee). But the amount of work with positioning and treatments (different than for dynamic speakers) was huge. To me that was the trade off. 

OP “…how tall should the stands be?…”

Good question… to bring the tweeters to your ear level in your listening position.

Stands are available at most audio outlets. The mark of good bookshelf speakers and stands are high weight. So, for a good speaker, you are talking 20+ pounds. For a high quality stand, high also. If you want to maximize the sound quality one should fill the stand with steel shot (in the old days it was lead shot). So together… you are talking about 40 or fifty pounds. So unless you are playing football 🏈 in your audio room there is no chance they are going to tip or be knocked off. I use vibrapods between my speaker and stands to decouple them from floor vibrations, and they are soft elastomer, so slipping is prohibited.