It depends! Is your girth (7" is impressive!) uniform from head to base? If it's big at the base but you want to increase it toward the head, then get a tube that's about 7" girth (2.25" dia). This will only allow the portion toward the head to expand. But, if your girth is already 7" from base to head, get a tube that everything can expand to fill (a 2.5" dia tube would allow all to expand to 8" girth.