First, choose a design and build it from the same lumber as your decking and railings. Otherwise it will stick out as a displaced design element.
If your built-ins are perpendicular to the joists, the joist spacing will affect the length. Benches built parallel to the joists can be any length. You can build both types on the same deck. It's easiest to frame built-in benches before the decking goes on. Bench seats should be about 18 inches above the decking and 15-30 inches deep.
Built-in perimeter benches help define the edge of a deck in addition to increasing the seating. For continuous seating, cut enough supports to install one every 4-5 feet. Most building codes permit perimeter seating without a railing on ground-level decks; on an elevated deck leave enough space for a railing behind the seating.