I've got my copy of "Electrical Wiring: Residential" 18th ed, complete with all the information, tables, and formulae for choosing electrical box sizes, but I've got to imagine that experienced electricians aren't spending 10 minutes per box calculating the proper size, and they're still meeting code. Are there any industry rules of thumb for looking at a circuit diagram, and quickly choosing the correct box based on what's going in it? I'm mainly interested in straightforward circuits comprised of receptacles and two- and three-way snap switches with standard NM-B cable (14/2, 14/3, 12/2, etc.).
Every 14 ga. wire entering (or exiting) the box needs 2 cubic inches. Every 12 ga. needs 2.25 cubic inches. All grounds together count as 1 wire, as do internal clamps. Every device (such as a switch or a receptacle on a single strap) counts as double the wire size it connects to.
Rough add-up (" standing for cubic inches):
One 14/2 cable in (2x2"), two 14/2 cables out (4x2"), ground wire (1x2"), two single pole switches using 14 ga. wire (2x2") = 18". Virtually any box could handle this.
As more circuits pass through, 3-ways are added in, the numbers add up.
In sum, every 14/2 coming in costs 4", every 12/2, 4.5". Every switch or receptacle counts for the same. You also need to account for ground and clamps.
At some point it becomes somewhat intuitive. It only becomes a problem when you are trying to cram a lot into a little space (like I am in this question)