The type streamoff is a synonym for one of the signed basic integral types of sufficient size to represent the maximum possible file size for the operating system.297
The type streamsize is a synonym for one of the signed basic integral types. It is used to represent the number of characters transferred in an I/O operation, or the size of I/O buffers.298
Typically long long.
streamsize is used in most places where ISO C would use size_t. Most of the uses of streamsize could use size_t, except for the strstreambuf constructors, which require negative values. It should probably be the signed type corresponding to size_t (which is what Posix.2 calls ssize_t).