text: Check your display's settings.
  If the DeX program doesn't fill the screen or looks distorted, you may be able to adjust your display to work better with DeX. If your display's native resolution is not 1080p, you may need to adjust the screen fit options or disable picture enhancements to get a clear image. Refer to the user manual or your display's manufacturer.
  
  Some apps may not allow changes in font size. The settings may only apply to the DeX screen.
  
  Change the screen and text sizes.
  You can edit the DeX screen size and text size if things look too small or too large for your display. In DeX mode, navigate to Settings, select Samsung DeX, and then select either Screen zoom or Font size. Adjust the slider at the bottom.Font size settings in the Samsung Dex app
  
  Try using another device.
  Connect the Pad, Station, or cable to another monitor or TV.  It is possible that your display doesn't support the resolutions required by DeX.
  
  Check the phone's compatibility.
  DeX is only available on select Samsung phones, including the S8 and up. However, it is not available on the Galaxy Z Flip. If the monitor doesn't do anything when you plug in your device, the phone may not be compatible.
  
  How to change the screen and text sizes ?