I posted this in another thread about almost the exact same topic.
"Welcome! Getting to understand bonsai can take some readjustment of popular ideas of what they are and how they're developed.
First--FWIW, small bonsai generally don't grow up into big bonsai. Big bonsai with substantial trunks are typically "cut down" from larger trees where the top branching is re-grown. Growing from seed is the longest path to a final bonsai
A foundation fact for bonsai is that once containerized, the tree's growth is greatly restricted. Seedlings of the size you have are not really bonsai material, per se. They're candidates for sure, but they have a considerable path to follow before they're ready for bonsai culture. The first step is to develop the trunk. Depending on what you're after (there are several "finished" sizes for bonsai, from mame--
only a few inches tall, to
trees that take four people to lift them) The "final" size (bonsai are never really finished, since they continue to grow), depends largely on the size of the initial trunk you're working with--anything above the first six to ten inches of trunk is largely expendable over time. The initial six to ten inches of the trunk is crucial for the final image you're after. It is the foundation on which everything else is built. That can be particularly true of oaks. Trunk development can take a decade in a container to get something impressive. A few years of unrestricted growth in a container may get you a trunk that might be able to hold up the leaves on an oak or chestnut. A smaller bonsai pot will slow that even further. Tap roots should be shortened before containerization--it's too late to do anything about that since root work is done in early spring before leaves emerge.
So now you're working with seedlings that need a lot of growth to be of much use for now at least. They need to simply grow and expand, get substantial, strong root systems developed that can fuel further development. The "final" diameter of the trunk depends on what you want to do. Also, small trees require more attention and care than relatively larger ones--soil mass, foliage load, etc. become more crucial as containers get smaller--small pots with their small soil mass dry out faster and can be more vulnerable to weather. Your tree should be outside its entire life. Bringing it inside with lead to a slow death. It will have to be overwintered which can be done by mulching them into a garden bed once leaves drop in the autumn."
