When creating content it is very important to adhere to our recommended dimension sizes before you upload the content to your NowSignage account.

https://nowsignage.freshdesk.com/support/solutions/articles/44002545398-asset-guidance

The reason for this is that if you exceed the recommendations, your hardware may struggle to output  and scale the large content. This can lead to issues with your device and can also cause screen sync to fail.

On the other hand if the content is too small, then this also puts strain on your hardware as it will have to stretch the image (or video if you have stretch videos set up in your screen settings) each time it is rendered on the screen.