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limiting pixel dimensions (with Imsanity) to whatever the max for the theme is. Filesize will take care of itself for the most part. You could add EWWW or other "smushing" plugins to compress and optimize images too.

https://wordpress.org/plugins/imsanity/
https://wordpress.org/plugins/ewww-image-optimizer/
https://wordpress.org/plugins/wp-smushit/