I have made a specific Scripture Theme.

It has a picture of a bible as the background with the text box at the bottom. The text box actually starts at about the middle of thee screen and finishes a little above the bottom of the screen. I then set that text box to be Bottom Aligned. That theme is set to be the default Scripture theme. Whenever I use Scripture that screen always comes up with the text as a lower third. Of course, as you say there are short texts and long texts. The text box is big enough for the longest Bible text and because it is set to Bottom Align, it alway runs along the bottom. As the amount of text grows, it grows to fill the text box. Of course no text is ever missing. I set the text to be a spcific siz and turned off Auto Sizing to fit the text box.

IN the three attached pictures you can see the result. The texts are just random grabs to show different amounts of text.