Feed Size
Control the number of posts displayed on the front page.
Go to the Layout tab:

and edit the Blog Posts gadget.
Adjust the number of posts on the main page
and click the "save" button in the bottom right.
To customize the thumbnail's height and styling,
go to the Blogger Theme Designer and
select "Advanced" ⟶ "Home Page - Blog Feed."
go to the Blogger Theme Designer and
select Advanced ⟶ "Home Page - Blog Feed."
To customize the parent container's background color,
go to Advanced ⟶ Backgrounds ⟶ "Main Body"