This is normally in .Xresources:
$ cat .Xresources
Xft.dpi: 130
To change it on the fly:
echo "Xft.dpi: 130" | xrdb -override -
Now restart some applications to see the effect. Some will change, some will not. For example, if you restart i3 (normally you do that with $mod+Shift+r) then you should see that the window titlebars have changed the size. For gnome-terminal however you're better off changing the font size.