Question about making a baby tutu?

I want to try and make a baby tutu. I want the tutu waist to be ribbon for 0-6 month old baby. About how long of a ribbon do I need to attach the tulle to? I know I am suppose the measure the size of the babies waist but this is for general use. I know I can just tighten it up some for smaller babies so just a general guide for a 0-6 month old baby.