@Usman Hussain, Thanks for posting in Q&A. Based on my researching, there's no built in policy to set the Default font size for Microsoft 365 app in Intune. After researching, I didn't find the method in Microsoft 365 either. Please ask in Microsoft 365 community to confirm if there's such option to set the default font size. If yes, you can ask if it can do via PowerShell script or registry key.
If yes, as a workaround, we can consider deploy it to the devices we want via PowerShell script via Intune.
Thanks for your understanding.
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