This question already has an answer here:

I am not just asking how to change the DPI of X11 as a whole, but instead how to make each monitor run on a separate DPI. I am trying to use two 900p monitors (19 inch) and one 720p TV (30 inch).

marked as duplicate by bain, Lucio, Eliah Kagan, Mitch Aug 3 '14 at 9:10

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Browse other questions tagged or ask your own question.