draw (oneself) up to (one's) full height
To stand up straight, often to project confidence or prestige. People would be more apt to take you seriously if you drew yourself up to your full height instead of hunching over all the time.
draw yourself up/rise to your full ˈheight
stand straight and tall in order to show your determination or high status: When the sales assistant said he couldn’t help her, she drew herself up to her full height and demanded to see the manager.