It's a long time since I bothered to check the laws of football, everything keeps changing and I've given up trying to keep up, especially the offside rule:-) As far as I know, the lines enclosing the penalty area are part of the penalty area, if the goal keeper's feet are touching those lines, he can handle the ball outside of that area. I think that would be in keeping with other laws that the lines are part of the field of play.
The ball and the goalkeepers hands are the only things that are important here. Both must be inside the penalty area. The line is considered inside. It does not matter where any other part of the goalkeeper is, so long as the ball and his hands are within the area.
Leachy is absolutely correct. The position of the feet is irrelevant when the keeping is holding the ball. He can step outside of the box, once he holds the ball inside. The boundary lines belong to the penalty area, so once the whole of the ball does not cross the line, he can legally hold, touch or parry the ball.