You can make subtitles larger. Got to Settings/Accessibility/Subtitles and CaptioningThey need something like this for text. Why does Apple choose the smallest font for all their text boxes? 50 inch tv and 3 paragraphs are smashed in to 3% of screen real-estate.
You can't exactly make general text larger (a strange oversight), but you can turn it into bold which helps many people with poor eyesight. Go to Settings/Accessibility/Interface.
There may be other settings in Accessibility that help you with specific issues...