To decrease your data storage, you should use the minimum size for your image. You can use the below size:
Header images: 1841 x 514 px.
Course, blog featured images: 870×580 px.
Slider Revolution on the homepage: 1920 x 1280 px.
Category icon images: 64 x 64 px.
Background image heading for category: 150 x 100 px.