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

Depends on the height of your speaker's tweeter (from the floor). Normally it will sound best when it is at ear level. Room and tweeter/speaker design might accommodate a slight deviation. For normal sized monitors we're usually talking about something between 24" and 28". 

  Start with the Cardas speaker placement guidelines. Then read all you can find regarding placement. Look at some of the speaker stand manufacturers like Sound Anchor. For entry level speaker stands check sites like Crutchfield and Audio Advisor. Items to consider are stand height and mass. Then consider how your stand will contact your floor ( spikes, sliders, pads ). How your speakers will contact your stands ( like Blue pad’s mentioned above ). BTW I’ve had speakers finish damaged by melting blue pads. As far as tipping over , consider kids, dogs, rowdy guests. When you get further into the learning curve you’ll be amazed at the sound quality that is achieved with high quality bookshelf speakers when properly done. Have fun and enjoy the hobby. Cheers , Mike. 
 

OP “…how tall should the stands be?…”

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

Blue Tack is the correct answer to your question (less is more when using it, IMO).

As far as the other comments go it depends upon the speakers you will be using.

For example "heavy" stands would most likely be a mismatch mated to speakers with lossy cabinet construction.

DeKay 

Solid stands are super important so they are not wobbly and loose allowing lost energy...not good. You want the speakers anchored to the stands with Blue tak or Posterboard putty ( this works really), and the stands you want no movement so each spike has to be adjusted (good stands have adjustable spikes or feet). In this way you will receive the full energy (which improves dynamics and Impact) and clarity of sound.

Matt M