How i edit Featured Image size

1. why photo different size and different resolution?

Answered in your following topic.

https://colorlibsupport.com/t/why-photo-on-site-is-different-size-and-different-resolution/

2. why I have 2 photo at homepage? I must don’t put first photo at Post space?

Yes you are right you must don’t put first photo at Post space so that 2 photos won’t display on homepage.

3. Featured image use for what?

It is used to display the image on the top of the post. Find more information about this here http://www.wpbeginner.com/glossary/featured-image/

4. If I share article,Featured image or first photo at Post space is show?

Mostly the Featured image.