I've updated the game to start in full screen automatically.
You can toggle between full screen and windowed using F11 or ALT + Enter

I've also patched the template saves insertion code. I shipped the game with a couple of template songs you can play around with and jam to but the game did not properly transfer the save files. I forgot to tell the game to first check if the save_data folder exists before getting it to copy over the save files.
I have patched this but this will only affect first time install users. If you've already attempted to play the game and it failed to copy the save files, it would not attempt it again because when it tries the first time, it created a flag file to make sure it only does so once instead of every launch. And in those users cases it would have created the flag file regardless if it failed or succeeded in copying the save file.

The good news is that you can reset the flag by deleting the initial_setup.dat file located in:
C:\Users\[your user name]\AppData\Roaming\Godot\app_userdata\Song Of a Spirit\

If you don't know how to reach this folder, you can also reach this by copying this into your folder viewer address bar:

%appdata%/Godot/app_userdata/Song Of a Spirit