This is where some basics of CSS are needed and it depends on if you are using my custom slideshow style or not. For mine, you would modify the inline styling of the captions. For example, this is the first slideshow caption off the demo site:
for Mobile Viewing
in Your Favorite Device!
Each line caption has a margin which you can change, colour of text, etc. You can add more CSS to the inline styles you see on each line as you need, including adding a font-size to the parts you want larger. But I will say you might have a bit of a challenge doing a top and centering, especially in relation to the responsiveness of the theme and slideshow. Again, this is where you will need some knowledge of CSS.