Welcome firewalker!
You can't select which kind of dimension is used.
This is done by LibreCAD automatically and depends on the arrow size, text size and dimension width.
Only when the text is too long for the gap between the arrows, the arrows are placed outside.
So you can modify text or arrow size, but that may bring other draw-back possibly.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution