You can right-click the text box and adjust the size setting. If you're on PS4, use the 'virtual mouse' mode (L1+R3 if I remember correctly) or plug in a USB mouse.

From memory, you can adjust it during 'normal-game' speaking to NPCs, but not in cutscenes. Once it's set, it should keep the setting permanently, although you'll need to redo it for any alts.