Hello,
Thank you for posting in Q&A forum.
Based on the information you provided, first make sure that enhanced session mode is enabled. If the problem still cannot be solved when enhanced session mode is enabled, I suggest that you open PowerShell as an administrator on the host and run the following command to change the resolution of the guest:
Set-VMVideo -VMName "Name of VM in Manager" -HorizontalResolution 1920 -VerticalResolution 1080 -ResolutionType Single
Note: You can change the resolution as needed.
Reference link:https://learn.microsoft.com/en-us/powershell/module/hyper-v/set-vmvideo?view=windowsserver2019-ps
I hope the information above is helpful.
Best regards
Zunhui
============================================
If the Answer is helpful, please click "Accept Answer" and upvote it.