Outdoor benches can be freestanding or built into the railing of a deck. Either way, it's important to get the dimensions right to create a comfortable place to sit. Without the right dimensions, your bench will last forever because nobody will ever use it. Test out benches in other locations. When you find a really comfortable one, don't be shy about pulling out your tape measure and taking some notes.
The length of a built-in deck bench should be as long as your deck. Since it won't be moved around, weight and maneuverability aren't concerns. A safety railing that's been turned into a bench looks better if it's consistent all the way across the length of the deck. If you're building a free-standing bench, make it no more than 72 inches long so you can move it from one place to another without too much difficulty. Also, free-standing benches look better when they're not too long.
The bench's seat height should be somewhere between 16 and 18 inches above the ground or floor. This is lower than a formal chair or a dining chair, but is appropriate for outdoor relaxing. Since you won't be using the bench at a table or a desk, you can afford to have the seat a bit lower and stretch out your legs. The seat's height is the main dimension that determines how comfortable your bench is. Design the bench so the back of the seat is slightly lower than the front.
The back of a bench doesn't need to extend any more than 24 inches above the seat. If you make the back too high, it'll look awkward, and it may obstruct the view if it's installed on the side of a deck. Build the back of the bench so it angles back slightly, providing a back rest that's compatible with the angle of the seat. Keep the angle between the back and the seat at roughly 90 degrees by tilting them both back at the same angle relative to the ground.
Most built-in benches don't have arms, but many portable benches do--usually one at each end--but some incorporate arms in the middle as well. Bench arms should be roughly 6 inches above the height of the seat, and should be made wide for comfort. An arm that's only 3 inches wide won't be comfortable. Make them 5 or 6 inches wide, providing a comfortable arm rest and a place to put your drink as well.