If you build yourself, get yourself a nice miniITX case and board. These will be nice and will fit your size requirements.
http://www.newegg.ca/Product/Product...d=1&Submit=ENE
http://search.ncix.com/search/?qcatid=0&q=Mini+ITX+case
It looks like the Alienware is using a customized silverstone case or something very similar
http://www.ncix.com/detail/silversto...94364-1711.htm
If you are looking for just a small PC and gaming/graphics are not a concern, you could always just get one of the small integrated solutions like this Zotac
http://www.ncix.com/detail/zotac-zbo...05390-1374.htm