Check out the option for Application Scaling (under the view menu) which appears when you are using the Standard UI in Sage 100 v2016+. This can increase font size although it's going to be an overall font increase versus just one field.

We are using Sage 100, not Sage 100c.

I think you still have the option to do this, even in Sage 100 (non C version).

There is no 'application scaling' option on my view menu for my non-C version (Sage 100 Standard 2016 (5.30.2.0))

Basic scaling was added for non-C in v2017.

To see the application scaling, you have to go to File --> User Settings --> And under "Desktop" change from "Classic" to "Standard".  I thought with Wayne's answer it might make the CustomerNo field bold but it still leaves it gray on gray.  But it is a larger font when you change the Application Scaling. I have to find out from the older users if they want everything larger, or if the bold/larger CustomerNo is really what they want.