I'm sorry to hear that, but unfortunately, any bigger and it'll take up more than half of the screen.
AFCL is a very text-heavy VN (or any other VN in general like you said) and is meant to be played on a computer screen.
The bigger the text are, the less lines we can show per window, and we don't want that. Having it too big also can be a little overwhelming.
As for how bright and thin, you can change the theme and give the text some outlines for better readability.
If, after having turned these features on, and you're still having trouble reading, then I'm sorry, this is my opinion only (pardon the pun), but I think you might get your eyes checked for some eye conditions—or play on a bigger screen and in a bright room, see if it mitigates that.
The game does tell you that.
I do appreciate your feedback! I'll keep this in mind and see if I can try to make the text bigger without affecting the game's design, but no promise.