Ship styles are made up from template ships. Yes, you can create and alter them outside the game like any data file, but changes will only take effect when you start a new game.
You can make new ship templates in the ship designer in-game, it will allow you access to all hull sizes.
When you have enough sjip templates to create your ship style, select your race as an opponent during game setup, and customise them. You should see a button to customise/create your ship style (it's always based on one of the existing styles). You should go into that and at the very least save as a new style.
You can either use the in-game chooser to pick your designed templates (remember to save/apply) or exit the game and customise the file by hand.