Hello,
1. There are no size requirements, it depends on your purpose. View our demo and check sizes applied https://woodmart.xtemos.com/
2. You will find the settings for the popup in the Theme Settings > Shop > Promo popup. You will find the shortcode in the field, navigate to Dashboard > HTML and edit the block.
3. One license – one domain. If one site has a live and testing version (staging domain) you can use one license for these two domains.
Best Regards