If you're sticking with the stock backing plate, you want 6.5" pads.
If you want to buy a 5" backing plate, then you can go with 5.5" pads.
The 6.5" pads tend to be thicker, and are (obviously) bigger. It's mostly just a tradeoff of how much space you work at a time, and price (the 5.5" pads...