It looks like a Pit Bull/some sort of Terrier mix.
Look at the eyes, they look a lot like American Pit Bull eyes.
Example:
But the ears, tail nose and body shape all say some sort of Terrier. Maybe some sort of smooth-coat terrier.
She'll probably be a good medium size, maybe 35-50 lbs. Not huge, but not one you can just tuck under your arm. And she'll probably have a lot of personality!
Good luck, we rescued our Rat Terrier/Spitz mix puppy from our local shelter almost two years ago. She's our baby (or at least will be until the real deal comes in March!)