There are many answers to this question – 1920px is one width and 1080px deep will fill most screens. The larger a picture the larger the image file. Chances are that for most websites more than one quarter of your traffic will come via a mobile phone, often 2/3rds to 3/4qtr. Google wants websites which work well on mobile phones so that goes against large file sizes as that will slow down your site. Smaller pictures could be 600px wide 400px deep.