Can the iPad Mail app show the message list full screen?
Sorry no, Mail.app on iPad only has two views.

In Portrait mode, the message list slides in from the left.  You can swipe left and right to change the view to account and folder list and back again, however you cannot resize the pane, although it feels like you should be able to in this view.

In Landscape mode, the message list is permanently fixed in position and width; you can no longer swipe to change the content.