I feel your pain, same here. I don't think there is any way to change the font unfortunately... had to play the game in small intervals and very close to the screen.
Actually, I have figured out a workaround if you are still playing! Hit Shift+A once in game and it brings up Ren'Py acessebility menu that allows you to enlarge the font or to use the sans type font. May look wonky in some cases (e.g save game info) but looks fine in dialogues and I was able to play with no problems! Wish there was a built-in version in the menu for that, though.