It's very difficult to read object description and cutscene subtitles when you're not close to the screen.
I didn't find any option to set the text size.
Is there a way to increase the HUD and the subtitle font size in The Witcher 3 on PC?
Currently there is no way of increasing this font.
A Patch was already confirmed by CD Projekt RED that will include a fix for the small text.
The details of the Patch haven't been disclosed as of yet. They will either increase the font or make it customizable. Now we have to wait for them.
Source: Gamespot News