You cant change this setting because the height of your display is less than 600 pixels
Users who are getting a new computer with the intention to install Windows 7 or Windows Vista Service Pack 2 need to ensure their display resolution is not lower than 600 pixels in height. Microsoft has lately revealed that if the display resolution is lower than 600 pixels vertically, Microsoft worries the DPI (Dots per Inch) Scaling will not function properly. The system will ignore the change request if users try to change the system DPI Scaling.
“When you change system DPI Scaling from “Control Panel > Personalization > Adjust font size (DPI)” on Windows Vista, or from “Control Panel > Display” on Windows 7, you may notice that the displayed font size does not change,” Microsoft has highlighted. “When the DPI Scaling change is requested by users, the system will check to see if the screen height is over 600 pixels. This is done to reduce the chance that dialogs and other user interface elements become inaccessible due to lack of screen real estate.”