Elisabeth Christine was born in 1715 so this has to be someone else of the 1720 date is wrong. However, she wears vestigial fripon curls, the sleeves appear to have cuffs rather than engageantes, and the crown is not Prussian. Based on her dress, this is not Queen Elisabeth Christine of Prussia. This portrait may one of Elisabeth Christine's mother, Antoinette (Antonia) of Brunswick-Wolfenbüttel.
According to her Wikipedia article, "Antoinette of Brunswick-Wolfenbüttel (Antoinette Amalie; 14 April 1696 – 6 March 1762) was a Duchess of Brunswick-Wolfenbüttel and the wife of her father's cousin Ferdinand Albert II of Brunswick-Wolfenbüttel. She was the mother of (Elisabeth Christine) the Queen of Prussia, (Sophie Antoinette) Duchess of Saxe-Coburg-Saalfeld, and (Juliane Marie) Queen of Denmark and Norway..." By Duchess Sophie Antoinette of Saxe-Coburg-Saalfeld, she was and is the ancestress of Duchess of Kent Victoria, Queen Victoria, and the House of Windsor.
Size fixed at 50 cm high at 28.35 pixels/cm.