I have the preview pane below the email list and they are sharing the vertical space about 50/50.  When an email is long enough to require scrolling in the reading pane, the scrolling in that pane is very choppy -- I don't believe this was the case before my update to Mavericks about 10 days ago.  I have applied all available Office and OS updates and rebooted.  I am using the trackpad and the 2 finger gesture to scroll, in case that makes any difference.

Is anyone else seeing this behavior?