Depending upon the thickness of the block and any other walls adjoining the block, Jim probably meant to say at least a 2by6or8 or bigger. Use treated lumber next to the block and the floor. Use tapcon concrete screw or metal anchors made for block. The plastic anchors would work but I try to use something more substantial for an anchor.
Also if it is going support a lot of weight above the door where you are going to add some big headers, then consider a double stud on each side to better support the weight. Good luck.