How to automatically scale a texture rect within a container?

I want to do this:

But so far the comic panels just extend full size instead of respecting a default size.

I know it's something so simple but I've been googling, checking youtube and reading the documentation for an hour and still don't know what should I be looking for.

I've tried: Changing the size flags of all the nodes to see what works, but nothing.

A little bit of guidance would be greatly appreaciated.