Well it would be impossible to eat yourself completely, but for the sake of argument though let's say you could. You couldn't "disappear completely" as what you're eating has mass and doesn't go into some black whole of non-existence. Also, you wouldn't exactly "shrink" either. Shrink is a horrible word to use for this question of yours by the way. Say you ate your arm however, and now it's completely down your stomach, away from human eyes. You now take up less space because the arm is inside your stomach, a previous sac of nothingness now occupied with your arm and hasn't altered your volume in any way. You could argue that you've reduced the amount of area you take up, however, that mass is still there. The area that your arm occupied is essentially transferred inside of you, as the area of that stomach is now occupied.