email viewing pane

For some strange reason my email viewing screen only shows the email on half the screen. I had to "drag" the right side "margin" on the screen's left side that shows the sender and subject of the email. When I click on it, the message opens on the right side, taking up the rest of the screen. For some strange reason the open email won't open all the way to the right, it stops at about the middle of the screen. I can't seem to find anything in "settings" where I can make it bigger. This is very maddening. Can anyone help?