Hi,
1. Do you get any error message when you try to change the font size on the computer?
2. Have you made any recent hardware or software changes to the computer prior to the issue?
You might be facing this issue because of an outdated or corrupted display card drivers. I suggest you to install the latest display drivers and check if it helps.
Method 1:
Step 1: uninstall the display drivers.
a. Press Windows + X, Click Device Manager.
b. Expand Display Adapters.
c. Check if the name of the graphic card is the list.
d. Right click on the Display Adapter’s name and click uninstall.
e. Once you uninstall the drivers it will prompt to restart the computer, do not restart the computer.
f. It will take default drivers in the computer.
Step 2: Reinstall the drivers.
Please download the drivers from the computer Manufacturer’s website and reinstall the drivers.
Method 2:
After installing the latest display drivers, I suggest you to refer to the following Microsoft articles to change the font size on the computer.
Make the text on your screen larger or smaller****
Internet Explorer Ease of Access options****
http://windows.microsoft.com/en-us/internet-explorer/ie-accessibilty-options#ie=ie-11
Hope this helps.
Please post back with the status of the issue and we will be glad to assist you further.