Horse chestnut trees tend to get rather large.. I have had a few for over twenty years in the hope that they may develop into a half decent bonsai. The problem that I found is that while the roots are allowed to grow the branches will grow quickly and long. That is with a long gap between sets of leaves and makes it look out of proportion. I have found that horse chestnut can take a lot of hacking it about, but the next problem is that when the tree gets rootbound, the branch growth is very minimal i.e.about 1/4 inch. This makes it very unattractive for bonsai . If it is attractive now, put it in a pot and let it get rootbound so that you will have minimal growth and with a little bit of luck it will stay the same for several years.