Some of your decisions for deck framing may be based in cost and how hard the boards are to move around. These are good reasons, but decks are long-term investments, and they're not easy to rebuild, so it's important to choose wisely before building.
You may hear that adding another beam can make 2x8 or even 2x6 joists work, which may be true, but that doubles the number of footings you'd need to install, plus more connection points and labor.
The best joist size for a specific project usually comes down to their span ratings. The joist's "span" is the distance between two supporting points, which may be a hanger, drop beam, or a footing saddle. For example, a 12x12 deck with one drop beam 10 feet out from the house and joists attached to the ledger with joist hangers, the joists would be spanning 10 feet, and have a 2 foot cantilever over the front beam.
Most treated lumber is Southern Yellow Pine, so for the purposes of this conversation, we'll use that species as our example.
2x6 joists
Best for: Clear spans under 8' when joists are spaced 16" on center. 2x6s also have a low profile, so you can build a low-elevation deck easily with no digging and no extra stairs.
Since the distance from the deck surface to the ground around the deck can be built to less than 7-3/4", the step off the deck would be the same or less than a standard stairway step.
The largest deck you could build with a triple 2x6 flush beam, 3 posts, and 2x6 joists spaced 16" O.C. would be 8 feet out from the house and 19-1/2 feet wide.
2x8 joists
Best for: Clear spans less than 10' when joists are spaced 16" on center.
2x8s are a common size for small decks, but won't produce as solid of a frame as larger joist sizes.
The largest deck you could build with a triple 2x8 flush beam, 3 posts, and 2x8 joists spaced 16" O.C. would be 10 feet out from the house and 21 feet wide.
2x10 joists
Best for: Clear spans less than 15' when joists are spaced 16" on center, or clear spans less than 16' when joists are spaced 12" on center.
2x10s are the most common sizes for deck joists, since they are also ideal for triple beams,
The largest deck you could build with a triple 2x10 flush beam, 3 posts, and 2x10 joists spaced 16" O.C. would be 15 feet out from the house and 21 feet wide.
2x12 joists
Best for: Clear spans less than 17' when joists are spaced 16" on center, or clear spans less than 19' when joists are spaced 12" on center.
In most common deck sizes, 2x10s are a great joist option. Big decks are often made by widening a medium-sized deck, but if an extra few feet is desired on the deck depth, that's when 2x12 joists come in handy.
The largest deck you could build with a triple 2x12 flush beam, 3 posts, and 2x12 joists spaced 16" O.C. would be 17 feet out from the house and 23 feet wide.
Final Notes
These guidelines were calculated using #2 SYP qualities, and may not be what your city has approved for deck framing, so check with your local permitting office.
If you're concerned about deck "bounce" or giving the deck a solid feel, you can easily space your joists closer together or upsize them.
Utilizing a drop beam and cantilevering your joists over it will add some depth to your deck with a given joist size, but will add labor, materials, and lose some stability. When using a flush beam, it is securely fastened to the ends of every joist, all of the front rail posts, and supports your railing. This integration adds strength, but does come at the cost of joist capabilities.
If you're going for something bigger than any of the listed examples, there is flexibility in your joist spacing, beam location, additional beams that can be added, or using treated LVL material for joists and beams instead.
