I believe that EETV Box PRO doesn't provide the option to choose the size or positioning of its linear broadcast channels subtitling. You simply have the option of subtitles on or off.
Some of the apps provided on the box do offer this facility which you select within the app, For example BBC iPlayer and Prime Video do. You may need to make the subtile size selection once you are watching the video.
When Youview started the partners were committed to providing accessibility features. Unfortunately that doesn't seem to a priority these days. The EE TV Pro remotes have a smaller number of buttons compared with legacy box remotes . Subtitle/Delete button was one remote button that was considered unnecessary. I suspect they prioritised frequency of button use in their decision making but making people who need/ wish to use those accessibility choices make extra button combinations doesn't seem the right way.
The more button does exist with an onscreen option to use subtitles, audio description, and zoom but and it is a big BUT the more button only functions on some screens. If using the apps on the box you need to learn where each app has its subtitles options listed.
Could/Should be better accessibility options and more helpful documentation.
