invalid reference image arkit 1.5

I am running the apple sample project of ARKit 1.5 Image detection but when I place my reference image inside AR Resource folder and run the app I get following error:

Make sure all reference images are greater than 100 pixels and have a positive physical size in meters.

I don't find any predefined requirements of how should the image be, can anyone explain the required attributes of the image?

As in Apple's documentation

For each image, use the inspector to describe the physical size of the image as you’d expect to find it in the user’s real-world environment, and optionally include a descriptive name for your own use.

Click on your image and chose size inspector to provide size. :)