The size of the stand you choose for any particular Bonsai or Suiseki ultimately comes down to what you prefer, however, generally speaking some negative space around the bonsai or stone is a better look. As a general guide,if the stand is about twice the length of the Suiseki or bonsai pot, you will be close to having a good balance.
Have a look at the three photos in the Post of a Suiseki on three different size Budget Range Stands, to get an idea.