I currently have an 8g ring in my septum, but I would like to stretch it to a 6 or a 4.  At what size does it start to droop on average?

And mine is pierced a little high, will that effect at what size it droops? 

it is different for everyone. some people can handle the bigger sizes with the droop. so it depends. as long as you don't wear something too heavy in it and you stretch carefully and give enough healing time, you should be fine to be at that size