Hi Stephanie,
Here is the answer of all question..
1.Blog layout same as full blog post.
There is no blog layout which looks similar as full blog post. You can select one from the four Blog summary style.
2.Showing up excerpt to 400 char.
You can do this by clicking on excerpt on Content or Excerpt . Then you can input Excerpt length to show number of words.
Go to dashboard ->Appearance -> customizer -> Basisc setting
Note: You can divide your content simply by adding read more tag on to the post. So Blog page will show just content before the read more tag. See here for reference:
https://en.support.wordpress.com/more-tag/
3. Extend image to full width of content.
if you place image of minimum 845px it will cover whole area and comes same as content width.
Let us know if you got any problems with this.
Thanks