Background picture size

I'm sorry, but would you mind providing us with more details together with your concern so we can further assist you?

If you are only asking for the specific image size, there is no specific recommendation / limit since we allow large image uploads, and also the image size may be quite different based on your goals. However, we recommend you to use optimized images due to performance reasons. For reference, please refer to the link below.

Google Developers - Image optimization

Looking forward for your response.