Depends on the wig, really. Some brands run elastic around the cap, in which case you might get away with an 8/9, but if they're not elasticated, an 8/9 could just roll around on the head. Try and find out of certain companies run 'big' or 'small', and make a choice from there. I find it's usually best to go larger than smaller, depending on the style, because at least then you can attach the wig so it doesn't fall off, instead of having it not fit at all!