Hi Andrew,
The incredibly small preview pane is a known bug we are working to resolve. I'll let you know when we have a fix out.
As to the preview capability you toggled, there should be a difference in functionality. When that capability is turned on, the item page loads and previews the document in the preview pane. When it is toggled off, then the document will load directly from the search results page, as DC has chosen to do: http://opendata.dc.gov/search?collection=Document
Try toggling again and reply back if it's not working as expected.
Best,
Patrick