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