If the subtitle format is a picture-based one (e.g. VOBSUB, PGS) then you cannot change the look or size of the subtitles at all.
The same applies to many text-based subtitles in the ASS format.
Some players do also adhere to the “color” and “size” formatting instructions which you sometimes can find in SRT subtitles, while other players ignore them. To get consistent results, I recommend you to remove these formattings from SRT files completely. See Introduction: convert image-based subtitles to SRT with Subtitle Edit for a viable method.
