If you do that, people using Landscape mode will be stuck with a 320px wide layout. That's not web friendly or responsive.
To change the default settings, you'll need to modify your CSS code manually. And I don't recommend it.
I'm not suggesting changing the site layout, just the button preset so that the document window shows 320 px wide when I click the Mobile size button. This is about modifying my workspace, not modifying a layout.