hippos can open there about 3 feet or more depending on the size
They open it very wide.
You can't. You can only open your mouth as wide as your jaw will let you.
the hippo would win because it has a big mouth.
The hippos mouth is 4 feet (1.2m) and its teeth are big too. See: Hippopotamus - The Hippo's Teeth And Its Diet for more information.
Open the mouth wide.
A grown hippo usually has a mouth about 4 feet tall.
Birds
A hippo has the bigger mouth. Tasmanian tigers were about dog-sized, while a hippo's mouth is about the length and width of a mid-sized suitcase.
roughly 65 degrees
Just as I opened my eyes, the shark's mouth gaped open right in front of me. The gate suddenly swung as wide as a mouth gaped open.
A hippo's tooth is about 13 in. wide. :)
Wide open. A gaping hole is a wide open hole. A gaping mouth is one which is hanging open, usually from astonishment.