Follow these few steps in order to make Caller ID appear on fullscreen on iPhone 11.
First, Go to the custom contact app on your phone
Second, tap the contact you wish to appear on fullscreen mode on iPhone 11
Third, tap or select the edit button on the top right corner of the contact you have selected.
Fourth, another edit button will appear just under the contact profile image at the top-left of the screen. Click on it.
Lastly, you will be required to choose photo on the button that appears. Modify images to be viewed in portrait.