Original: 13.5MB
6000 x 4000px

Compare quality of WordPress/Smush Optimised: ~220Kb

When image is uploaded to WordPress image is resized automatically

That said if you optimize to say 1600px wide at medium quality the file size reduces to 158KB

My rule of thumb is that a full screen size is say 1600px wide so if you want an image to be about ½ the page I will make it 800px wide and a ¼ page is 400px wide.
(It does not have to be precise but good ‘rough guide’).
I usually save images at medium quality as anything higher is not usually noticed on a normal screen resolution. Hope this helps a little.

To view this reduced size image, click here to see download speed difference
Quality difference – negligible