I had this problem and instead of using the default "x" for pvi's I used a standard block (ours is a triangle, but you could just use and "x"). Doing it this way ensures that it isn't seen as a point, but rather an object and it will scale properly for half size plots. (go to the curve label styles, add your block there. then set the anchor point to anchor pvi, set the scale and you should be good to go.)