Expanding hull is done with the hull expansion tool, found in the build menu. Hull can be designed with the hull sketching tool, which will only sketch hull tiles but not yet give the command to your crew to build them. You can use this tool to create a larger design before you start to actually build the hull. Sketches can be activated with the hull activate sketch tool, found under the same menu.
If you don’t want to sketch anything you can use the build hull tool, which will immediately give the command to your crew to start building new hull tiles. Note that new hull is built with mining pods, and will require hull blocks. As long as you have the required resources (Hull blocks) and some crew members assigned to construction the hull will eventually get built.