I hard to see when the scale is small in xfce4 default setting.
Therefore I was changed scale on my display.
Settings Manager → Appearance → Fonts, and change the DPI parameter.
Best size is 120 for VAIO S11
Go to Settings Manager → Appearance → Fonts, and change the DPI parameter. The value of 180 or 192 seems to work well on Retina screens. To get a more precise number, you can use xdpyinfo | grep resolution, and then double it.
To enlarge icons in system tray, right-click on it (aim for empty space / top pixels / bottom pixels, so that you will not activate icons themselves) → “Properties” → set “Maximum icon size” to 32, 48 or 64.