Text size is controlled by re-sizing the box around it rather than giving it a size as you would in Word. That's because you can vary the Zoom on each image and therefore setting text at 12 throughout wouldn't mean it showed the same size in each screen.
There is a center option either I'm afraid. If you need to organise text in that way you could use separate boxes for each line and position them to match
How do you select font size to ensure text in different text boxes are the same size?
How do you center text in a tex box?