Draw yourself up/rise to your full height
Definition 1 of 2
Pro Tip 1/3Confidence Display
The phrase often shows one's self-assurance and confidence in a challenging situation. 
Challenged by the debate opponent, she drew herself up to her full height and spoke with conviction.
Pro Tip 2/3Physical Stance
It literally involves straightening your back and shoulders; the action complements the feeling. 
He drew himself up to his full height as the national anthem began to play.
Pro Tip 3/3Assertiveness
Used when someone displays dignity in response to disrespect or criticism. 
Despite the scorn, she drew herself up to her full height and exited the room proudly.