No, it is not possible to customize the font settings in Duo for Windows Logon. It is possible to use a screen magnifier or other accessibility tools available to Windows to view larger.
Please contact Duo Support or your Duo Care team/Cisco account reps to submit a feature request for larget font or customizable font size.
You mention "Where it use to be big font for the code display." I think you might be mixing up two different products and login experiences.
Verified Push in Duo for Windows Logon is a new feature; there has never been any other font size used for messages.
Duo Universal Prompt, seen during browser-based logins to applications, also supports Verified Push, and shows the verification code at a larger font size. This experience is unchanged.

Duo, not DUO.