Seven foot frame usually means the stature of a person who is seven feet tall so approximately 213 cm Hope this helps!!

Bruno is being described at that point as a scary figure. Seven foot frame is describing how tall he appears to be. 7-feet tall. While rats ride on his back as he’s mumbling and scaring the family. Thought not stated, I imagine being a shadowy figure, Bruno’s shadow may give the illusion he is 7-feet tall. Villainous characters always have a long/tall shadow.