Better question is how tall are you?
I have a 6ft bivy and 6ft sleeping bag (I currently use a quilt that might be shorter than six feet), but I am 5'6" tall.
I have found that I enjoy having a bivy that is longer than me to allow me to store much of my gear inside of my bivy above my head while putting my empty pack under my legs, inside my bivy or outside, depending on the weather.
If weight is your concern, go with the smallest possible bivy.
I personally would not, as I appreciate having the extra space inside my bivy to move around and to store gear.
Hope this helps.