Elias here with Shopify Support. Thank you for reaching out.
There is an option to enable wide layout for the Minimal theme, which would allow the Slideshow to reach full-screen (based on the width of the display or window). When using the wide layout, the text or font sizes are not affected. The setting to enable wide layout can be accessed through the Storefront editor, located in the Layout section of your theme settings.
Here's a short video on where to find this settings (using the Minimal theme from my test store):
Let me know if this is what you're looking for?