There's a dividing double line between both sections (between the message list and the message). When you hover the mouse over it, the pointer changes into a two-sided arrow and you can pull the divider up or down to change the relative sizes of both.

If you don't want to see the message list at all, use the menu: View | Show Message List | Hide.

ps: as for myself, I never double-click on a message (except by mistake). I use a window split mode (Workspace | Window Split Mode | Full Hight Account Tree) that shows the account tree on the left, and the message list and current message on the right (top 2/5th and bottom 3/5th respectively). That way I have all the information that I need on my screen at once.