Town Hall is the center of your Garrison community and is your main point of interaction. This building is automatically placed in your Garrison when you start it, and is placed in a set location.
The Town Hall is also where you get information such as how to advance to the next tier of your Garrison (currently, that is by having a building on every open building plot).
Because your Town Hall is automatically placed for you, it is not considered to be a small, medium or large building. Clicking on the Town Hall in the interface will give you more details, or you can visit it on your character.