An important question when one starts of with bonsai is: How tall should my tree be. Naturally you have read about the different tree sizes. But that tells you mainly what the different size-classes are. However, that does not tell you how tall an individual tree can be. Yet there are very clear guidelines to the size a tree may be, based on the diameter of the tree. The tree-diameter to tree height ratio helps to produce a realistic image of an old tree.
When growing bonsai, you try to bring across the image of an old tree. Typically, these trees are tall. So looking at it from a reasonable distance would distort the taper a little: The base of the tree would appear to be much wider than it actually is, and the taper is much more extreme too. Keep this in mind when looking at ratio’s below.
A typical bonsai will have a ratio of 1:6 to 1:12 between the diameter of the trunk just about the roots to the height of the tree. In other words: If you have a trunk diameter of 2cm, you can grow a bonsai of 12 up to 24 cm tall (trunk diamter of 2 cm times 6 = 12cm, 2cm times 12 = 24cm). Of course these measures are not cast in stone. Many trees are taller, and still look good. And there is a specific class of trees which is much thicker, the so-called sumo bonsai. These may have trunk diameter to tree height rations of 1:1.5!. However, in order to create a convincing bonsai, a ratio between 1:6 and 1:12 will help a lot.