Is my hamster crossed with a guinea pig?

That's genetically impossible. Guinea pigs and hamsters are two completely different animals. You definitely have a syrian hamster on your hands. What do you define as 'large for her size'? Syrians are generally 7-8 inches long. She may seem a bit fat but it's most likely her cheek pouches or she's on the larger end of the spectrum. Good luck with your new hammy!