Case 12'' X 13''

The 12" x 13" size corresponds to an Extended ATX (abbreviated as E-ATX) motherboard's dimensions.. So look out for this term while looking for a case to fit such a motherboard.. In other words, you are going to need a case with support for E-ATX form factor motherboards.. Here are few examples -

www.newegg.com/Product/Product.aspx?Item=N82E16811119225

www.newegg.com/Product/Product.aspx?Item=N82E16811112243

www.newegg.com/Product/Product.aspx?Item=N82E16811163180
 
Never heard of a mobo with those dimensions most of the big boards are 12"x10.6".
With the exception of maybe the MSI Big Bang 13.58"x 10.39".
These bigger boards are known as E-ATX and quite a few full size cases support that format.
Here's an example of just a few cases that support E-ATX(extended)mobos.
http://www.newegg.com/Product/Productcompare.aspx?Submit=ENE&N=100007583%20600006304&IsNodeId=1&page=3&bop=And&ShowDeactivatedMark=False&CompareItemList=7%7C11-146-068%5E11-146-068-TS%2C11-139-001%5E11-139-001-TS%2C11-119-160%5E11-119-160-TS%2C11-119-225%5E11-119-225-TS%2C11-163-180%5E11-163-180-TS