Configuring preferences and options in Outlook.com
Hi. I'm Brian and I will do my best to help you.
Since OWA is a web-only interface, there won't be any registry entries to control it. However, I find I can click on and drag the vertical line between each field and drag the edge of the field left or right, decreasing or increasing the field's width. Note this is with the Reading Pane at the bottom. Obviously you can't make the header line wider than the browser window.