We recommend to make images 1200-1600px on the bigger side; 66-76% JPG quality – this is a good compromise between image size/quality and page loading speed. Not bigger than 1700×1100 px for a fullscreen slider. If slider is not fullscreen, you can use approx. 1024px wide images.
Ideal image proportions are: 16*9…16*8. For example, 1040x585px, i.e (1040px/16)*9=585px.