Hi Macisback100,
Thank you for participating in the Apple Support Communities. We understand that your iPad will not show a full screen resolution when using Sidecar. We recommend making sure everything is set up correct by following the steps in this article: Use iPad as a second display for your Mac.
"Sidecar requires macOS 10.15 or later and iPadOS 13 or later on supported models.
Set up Sidecar
- Make sure you’re signed in with the same Apple ID on your Mac and iPad.
- Use one of the following connections:
3. On your Mac, do one of the following:
- macOS Ventura: Choose Apple menu > System Settings, click Displays , click , then select your iPad from the list below “Mirror or extend to.”
- macOS 10.15 to 12.5: Choose Apple menu > System Preferences, click Displays , click Add Display, then select your iPad from the list below “Mirror or extend to.”"
If the issue persists, Contact Apple Support for further assistance.
Take care.