Cardinals will NOT nest in a house. They only require a box. Floor should be at least 7″ x 8″. A little larger is OK. Back of the house can be 4″ high with drilled holes so you can attach to a tree. ( They love the evergreen trees so if you can get in there put it at least 8 ft above the ground and settle it on a branch or 2) The front peice of the nesting box only needs to be 2 – 3″ high, so the babies won’t just fall out. The Cardinal LOVES sunflower seeds so sprinkle a few in the box when you set it in the tree.