This command could be required by some servers to reserve
sufficient storage space to accommodate the new file to be
transferred. The Size must be specified by a decimal integer
representing the number of the logical byte size of space
to be reserved for the file.For files that can be appended a
maximum size can be specified as well.
This command is normally followed by either the STOR or APPE commands.
The ALLO command is treated as a NOOP (no operation) by those servers
which do not require the file size to be specified by the ALLO command.