Multi-line layout is only available in a table view type.
Change the number of characters displayed before multi-line layout switches to single-line layout
You can specify the width (in number of characters) of the message list before Microsoft Outlook switches to single-line layout.
Specify the number of lines that display in multi-line layout
If the message list in multi-line layout is not wide enough to display all the column headings about an item, pausing on the item will display a ScreenTip containing all this information, plus any additional fields that are displayed in the single-line layout for this view.