Choose Edit and in the Content tab, select which articles you want to display.
Then, in the List Settings tab, choose how many articles to display, the order and the list template (how multiple articles display on a page).
Lastly, in the Single item settings tab, choose between 2 detail templates. This is how individual articles display once site visitors click on them.
The default behavior for the detail template is "Article", which allows you to use whatever image size you have added as is. The other option is the "Article Hero" template which includes a highlighted horizontal full-width hero image.