when you say 5 1/4 caddies do you mean these ?
http://www.overclockers.co.uk/showproduct.php?prodid=HD-037-BT&groupid=701&catid=14&subcat=72
or do you mean the ones you put a dvd drive in to make it an external ?
HDDs are either 3.5" (desktop) or 2.5" (laptop). I've never heard of a 5.25" HDD before.
The caddies are here... http://www.overclockers.co.uk/productlist.php?groupid=701&catid=14&subid=72
5.25" is the width of a case bay that you put your DVD drive into. Just an FYI.