I was paid a sign on bonus and signed an agreement stating I would repay it in full if I quit within the year. I did quit before the year. How much, if any, can they take from my final check?

In my state, they can take everything but minimum wage times the number of hours you worked. California may be different. Check with your state DOL.

You did breach the contract and due owe them the money. The question is can they just take the money from you or do they have to sue you in small claims. I say they have to sue you in small claims. Why did you quit? Was it because you found a better job or was the work enviroment unsafe?
Even though we live in a tyranical republican time in history we as good Americans have an obligation to be ethical even when the CEO's are not.
Nobody defends the little guy in America. Alot of the advice you get here is from slimy coporate defense attorneys who are starting to believe there own lies as truth because they repeat them so much.