The suggested height of your pergola is dependent a little on the total size, with larger structures being taller. Having said that a normal height would be in the range from 7' / 2.1m up to over 11' / 3.35m depending on the size and the location. The minimum should be 6'8 / 2.05m which is the same height as a doorway. Other considerations to the aesthetics, such as the Golden Ratio (length is 1.618 times the height), may also help you decide as well as other factors such as attached Pergolas being below the roofline.