what is your average size bonsai (in inches), and why did you personally
pick that size. BRBR

Most of my bonsai are about 12 to 18 inches tall. One reason is that the
species chosen tend to work best around that size. Another reason for not
having bigger bonsai is that I need to be able to carry them. For indoor
bonsai, the main limitation is the amount of headroom between the shelf and the
fluorescent lights. I have a few small shohin, about 6 to 8 inches tall, but I
keep it to a minimum because the tiny ones are the hardest to care for.

when does a tree started from nursery stock generally move to the status of
Bonsai?

Ha! That question is debated endlessly. It depends a great deal on the species.
I would say a minimum of two years before you can put it in your club show. As
a rule, for something like a juniper or 'Seiju' elm which has been pre-trained,
it will look more or less like a bonsai almost as soon as you put it in a
bonsai pot & take the wires off. With something like a Japanese black pine, it
might take 20 years. If you look at a tree and you get that feeling that you
can shrink to three inches tall & walk under it, it is a bonsai.
That is one reason why Toshio Kawamoto developed the concept of saikei. Instant
bonsai, no waiting.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)