In this photo is a Suiseki on three different stand sizes to give you an idea of what looks good or not so good. The stand in the middle photo looks better than the one on the left simply because there is a little more negative stand space around the stone. If you wanted to improve the focus on the stone, you could opt for a slightly larger stand size, as in the photo on the right.
In another blog post, 3 Suiseki are placed on different size Jita for a comparison look.