How tall should I keep my bonsai???

That depends. Are you talking about just one tree or your collection?
The classic rule is that a bonsai should be six times as tall as the thickness
of the trunk at the bottom. However, very few amateur trees reach this ideal.
In many cases, those proportions would look grotesque. In general, the fatter
the trunk & the larger the leaves, the taller the bonsai.
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)