By default, the iPhone Mirroring window on macOS Sequoia appears at actual size and cannot be resized. The frame of the iPhone with iOS 18 is not draggable and the size of the iPhone cannot be enlarged. However, it is possible to make the window larger through a hidden setting.

Make sure iPhone Mirroring is closed, then perform the following steps:

1. Open Terminal. It can be found in the /Applications/Utilities folder.
2. Run defaults write com.apple.ScreenContinuity showScalingControls -bool true. Copy, paste and press Enter.
3. Restart iPhone Mirroring. Quit the app and reopen it.
4. A new slider is present in the title bar. Drag it to zoom the window larger.

This sets a hidden setting in the ~/Library/Preferences/com.apple.ScreenContinuity.plist file which makes an extra slider appear in the title bar.

Drag the slider to make the window larger and zoom the contents.

To reset this functionality and remove the extra slider, delete the override on the setting using defaults delete com.apple.ScreenContinuity showScalingControls and restart iPhone Mirroring.

Hopefully Apple adds proper support for resizing the window in the future, but for now this workaround works!