Featured image and a few media questions

For some reason, my featured image appears larger on one of my sites. The images do not fit 100% in my browser window, whereas they do on my first site. The settings for both sites are nearly identical.

1. Where is the setting that controls the size of the featured image?

Also, in Settings > Media > Image Sizes, I have the following settings:

Thumbnail size: 150 x 150 (that’s fine)
Medium size: 300 x 300
Large size: 1040 x 1040 for the site with smaller featured images (1024 x 1024 on the site with the larger featured images)

Is there logic behind the Medium and Lage image sizes? My usual max-width for images in the blog posts is 450 px. I create featured images with the max-width of 600 px which normally has a height of 400 px.

2. Should I change the Medium medium setting to 450 x 450?
3. Should I change my Large size to 600 x 600 since I create max-width featured images of 600 px? Why 1040 or 1024?
3. What is the Large size setting for? I do not upload anything that large.

The page I need help with: [log in to see the link]