Amazing game I've just finished it and overall introduction is great same goes to the art and background music and pictures I sooo look forward to the updates !!
If you want to test dialogue boxes you should double click somewhere on the screen when picking a dialogue box to save then test out the different dialogue boxes because I don’t think you can change the boxes half way through the story.
You can change it on the setting screen by clicking "Simple Texbox". If that button is highlighted, you have the bigger textbox. If that button isn't highlighted you'll have the smaller textbox.