Largest internal hdd?

Hi guys.
I have a 500gb internal hdd in my ps3. Its getting filled up. What is the biggest i can go internal?

Im on rebug 4.21. Came from kmeaw 3.55. I never downgraded. Then if i can go bigger. Can i just put the new one in. Format it install rebug 4.21 or do i have to downgrade first. Install 3.55.

I dont want to do external cause theres always issues.