Hi Shinji Esaki @Shining esk. ,
You can change the size of the default Cube to any size you want (and make other changes) and then save the File as your new Startup File (although I wouldn’t recommend changing it to something so small, you”ll probably have to scale it up almost every time). Make sure to Apply the Scale (in Object Mode CTRL+A > Apply > Scale), it should be 1, 1, 1:
