The Sedio 1600mAh Nexus One battery is the same dimensions as the standard variety. While 1600mAh (That's a unit of measurement for the amount of juice a battery holds. A higher number means a bigger battery.) is by no means small, the Nexus One is not known for it's great battery life. So you might want to consider keeping a spare on hand, just in case.
The 3200 mAh Nexus One extended battery really packs in the juice. It comes with a contoured black replacement door featuring Seidio's signature soft touch coating and a textured bar to mimic the phone's original design. This battery is designed for those who can't always get to a charger, or for the serious power user.