There isn't a way in-game to change NPC text size.

I solve the problem by turning on Windows Magnifier when reading NPC text. The game's Screen Mode has to be set to Borderless Windowed for Magnifier to work (System Configuration -> Display Settings).

Press Windows logo key + Plus Sign (+) to turn Magnifier on and Windows logo key + Minus Sign (-) to turn it off. A magnifying glass will appear that you can click on to open the Magnifier dialogue box. From there you can set the percent magnfication you prefer and whether the whole screen or only a small portion of it will be magnified. I've found that 150% magnification in Full screen works best for me.

The magnfied text is a little grainy but still very readable.