Inside the scene designer, select the canvas fit mode from right panel.

None option will display the images/videos with their original resolutions.

Adjust & Center (recommended) will fit either the width or height to the optimal value and place the image/video in center.

Stretch option will disrupt the media to cover the screen completely. It's not recommended if the screen resolution and media resolution is noticeably different.

Add an image or video to the scene.

Select from Fit mode drop-down menu on the right panel.

Keep Aspect Ratio makes the placeholder size equal to the media. When you resize, aspect ratio will stay the same.

Adjust & Center will set the media width or height to the placeholder and place the media in center.

Stretch will disrupt the media to cover the placeholder completely.