As far as the base goes, it depends on the weight. There's no recommended width-to-height ratio I'm aware of. For scratching posts, the base should be at least 1/3 of the height, but there we're talking about an item where the weight of the cat could be a large percentage of the weight of the scratching post. Whereas with a cat tree, the weight of the cats on it is a much smaller percentage of the weight of the tree. What I'm saying, is that a heavy cat tree is less "tippy" than a scratching post simply because of its size as well as its weight. I think more important than size of the base is whether the center of the weight distribution (center of gravity) is over the center of the base and whether it's in the lower half of the height. The cat tree I bought had the vertical structure offset to one side, so in that direction it was a bit tippy and placing it against the wall was the only solution.