Hi there,
Every time I start a presentation I have to struggle with the annoying feature that the font size in my textbox is adjusted so that the text fits the box (autofit text option). I know that this can be disabled by right-clicking the textbox in question and I have also heard rumours that this can be adjusted in the master slide.
But I would like to turn this option off FOREVER (and if it were me, get the lines of code deleted forever - having 20 different font sizes is the worst feature ever!!). I frequently use Impress to quickly throw together a presentation and it just costs too much time to adjust this every time.
I’ve seen various different entries from others struggling with this as well and so far haven’t seen a good answer. Maybe this time we can find a solution 
The property in question is Shrink text on overflow in the UI, command .uno:TextAutoFitToSize, file markup style:shrink-to-fit. This property is not available as global setting. At least I find no suitable item in the advanced Expert Configuration. You can only disable it in general, when you create an own presentation template, that disables it for the presentation objects. You should add the property to a toolbar, so that you can see whether it is enabled or not.
The Shrink text on overflow property is not available in the UI of the style of an object. So in your new template you have to set it as direct formatting on the presentation objects in the master slide. You have to do it for each master slide and each presentation object. When you then base your presentation on this new template, the automatic shrinking of the font size should no longer happen.
When you disable Shrink text on overflow, then you need to decide what shall happen if the text is higher than the object height. That is the setting Fit height to text in the Text Attributes in the context menu of the object, or tab Text in the style of the object. In case it is disabled, the text overflows the object rectangle; if it is enabled, the object height is expanded to the height of the text. The difference becomes visible if the object has a background or a border.