Whats a good trunk thickness?
Generally, a good rule of thumb is that the tree´s height should range between 6-12 times the trunk thickness. That is, if you have a trunk of 10 cms in diameter, the tree´s height would hover around 60-120 cms, dependind the style you pretend to apply to your tree, etc.
This, of course, is not a rule written in stone and you can bend it, but it will help you for sure to determine some good proportions your tree should approximately match, in order to look balanced.
Alain