You can change the size and style of your subtitles by going to the main Roku device settings. To do this, please follow these instructions:
1.Visit the Main Menu of your Roku device.
2. Look for the settings section and select Captions.
3. On this sub-menu you'll find many options for subtitles like text style, edge effect, size, color, opacity, etc. Please chose your preferred subtitles style.
Since this is a global setting, your subtitles will appear this way on all your Roku channels. You can change this at any time.