I found another part # that was closer to the factory 7 x 8" size, but it woldn't fit in properly, because of the vertical tabs inside the housing prevented it from going all the way in... plus it was a real pain to try and squeeze in around the blower moter.
The smaller one seem to fit fine, doesn't seem to be moving around and completely covers the blower opening and filters all the inbound air.
While Fram and Purolater do make "Cabin" air filters, they do not list one for the Impreza application... don't know why!!?? - so this is an "Engine" air filter...
I think cutting one of those "cut to fit" indoor air filters to size, is also a great idea. They offer better flow, but not as good filtering to keep all those dusty dirt roads out of your air vent... Anything's better than paying retail at the $tealer!