serve scaled images

Hello

I have a issue from the gtmetrix. Its says:

The following images are resized in HTML or CSS. Serving scaled images could save 385.6KiB (82% reduction).

https://ellenaskitchen.com/wp-content/uploads/2019/09/Ellenas-Kitchen-Cover2-845×684.jpg is resized in HTML or CSS from 845×684 to 358×290. Serving a scaled image could save 201.3KiB (82% reduction).
https://ellenaskitchen.com/wp-content/uploads/2019/09/Reservations-845×684.jpg is resized in HTML or CSS from 845×684 to 358×290. Serving a scaled image could save 168.6KiB (82% reduction).
https://ellenaskitchen.com/wp-content/uploads/2019/10/Color-logo-with-background3-845×684.jpg is resized in HTML or CSS from 845×684 to 358×290. Serving a scaled image could save 15.7KiB (82% reduction).

I can not solve this problem. I dont know what size I exactly of the images I need for my website ? How can I finde the out. I already try to finde out the proper size with the browser tool from chrom. But i still not clear about what size I need.

Thanks for your help
Kind regards
Roger