Well, I know for sure that I want to get a fatter trunk going. I am hoping to end up with a tree that is about 1 1/2 to 2 feet tall with an informal upright style. I want to have a very full canopy and a trunk that is about 5 or 6 inches around. All in all, I want a nice, healthy, aged looking tree with a lot of ramification.
Lets consider an 18" tree for the sake of this example. The classic proportion is 1/6 ratio between trunk diameter and overall height, which gives a diameter of 3". Pretty big Huh? To achieve a trunk this big will require several years of unrestricted growth, any talk of pruning or chopping now is way premature.
Also, the tree is already more than the desired height. At what point do I cut it in order to begin training?
Don't prune the top until the trunk has reached half to two thirds of the desired diameter, consider it a sacrificial leader. The tree could be 8' tall and all you will use is the first 6" or so. At this time the trunk is cut down to around the height of the first branch, in this case about 6" (in reality it might be best to allow for some die back). A new leader is then encouraged to grow out at an angle. This process creates both movement and taper. The new leader is again allowed to grow out unrestricted until a pleasing transition is reached. Repeat this process several times and you will have a trunk to work with. All branches will likely be removed or cut back severely and regrown so that they will be in scale.
Since you have decided on a larger trunk putting it in the ground on a tile, as Scott recommended, is an excellent idea.
Here is some reading: