Weird question. Hard for me to word, which has made it impossible to research. I also thing the answer is obvious but I need to make sure.
If I wanted a tree from a seedling to become a bonsai that is, say, 4 feet tall and 4 feet wide, do I grow it out to 4 feet tall and wide immediately and train it accordingly or do I train it as a 1x1 foot tree and over the 15-20+ years (depending on the species, sure) it will expand as it grows and retain the originally trained shape for the most part? My assumption is you train it in the size and shape you want it to be forever. The only growth will be extensions from new growth and width because of age as tress do.
As an aside, I imagine you could have a 1x1 bonsai and decide later to grow it out, but I assume the trained sections will stay the same shape and size forever unless trimmed and reset.