Looks like the Mini tower which is a Micro ATX.
Spec Sheet is here.
http://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/optiplex-990-spec-sheet.pdf
Here is a Pic
http://www.ebay.com/itm/Dell-Optiplex-990-MT-Intel-Desktop-Main-System-Board-Motherboard-LGA1155-DDR3-/122022639280?hash=item1c691db6b0:g:eLQAAOSwNuxXaY9Q