Most models have very unconventional looks and are disproportioned. So maybe Rihanna does have an oversized forehead. So does Tyra and many models have oversized ears, noses, lips, eyes, etc.
She's a 5 imo. Flawless skin and complexion, eyes, body... she has an unique look.
Lady Gaga I would give a 3. She's weird most of the times but she has her times when she's a HAWT mama... Only If I had that picture where she had blonde hair and was wearing a yellow dress with heels. It was like a few months ago...