the basic measurements i've always heard, and tried to go by are as follows:
for display, the depth of the pot should be as deep as the trunk is wide.
the trunk should be 1/6 the height of the trunk
the first branch should be situated at 1/3 the height of the trunk
the second branch should be situated at 1/3 the distance from the first branch to the apex
the third branch should be situated at 1/3 the distance from the second branch to the apex
and the apex foliage should start 1/3 of the distance from the 3rd branch to the top.
These are only guidelines, and can be dramatically altered in order to create really unique and interesting designs.
Also, from an aerial view, the 3 major branches should create a scalene triangle, where none of the sides or angles created are the same length.
also, here's a page with all the rules that i could possibly think of (it's not my page)
Hope that helps,
So neither he who plants nor he who waters is anything, but only God, who makes things grow (I Cor 3:7)