Home menu -> Settings -> Accessibility ->Captions -> activate first "display" and then do your changes.
ST is then changed in VOD, Series and Live TV. Hard-coded subtitles and/or from server set as default, are not changeable!
I already tried this. The TEXT SIZE transition between NORMAL and SMALL are too much and if there is an option to introduce a MEDIUM one that is between NORMAL & SMALL, that would be great. Or simply increase the SMALL Text Size by 1 or 2 more scales.