At least
twice the cross-sectional area of the conduit or tubing to which it is attached
[314.16(C)(1)].
Yes, but
only if said body is durably marked (by the manufacturer) with its volume
[314.16(C)(2)]. And it cannot be a short-radius body [314.16(C)(3)].
Those would
be Table 314.16(A) and Table 314.16(B). The first tells you the minimum volume and maximum number of
conductors for a given box trade size. The second tells you the volume allowance
per conductor.
They must
be adequately closed
314.17(A).
They must
be suitable for the lowest temperature-rated conductor entering the box [314.17(C)].
Also, be careful which temperature columns you use in those ampacity tables. The
lowest temperature rating of any conductor brings down the temperature rating of
all conductors enclosed in a box or raceway.