How can I resize featured image & make a coustom home page?

I am not sure how to go about solving these two problems that I am facing, my first problem is how do I edit the post layout such as the featured image so it is not so large? How do I make the featured image much smaller?

My second problem that I am facing is that I am not able to get a custom homepage to work. I copied the 404 page and made some modifications to the original and change the name to mypage.php for example and uploaded it to my WordPress website however when I go to access mypage.php I instead see 404 page not found even though I have uploaded mypage.php. I do not want to edit the 404 page, I want to use the 404 page is more of a temple that I can make a few tweaks to and use the newly created page as my new home page, so how do make a newly created page as my new home page showing a search box that is center of the site and below that on the left side it has an area where I can paste custom HTML for example like breaking news and on the right side displays five or six recent post?