While GetSize in general is not a const function, it can be on a MemoryMappedFile. I propose to add a const override directly to the MemoryMappedFile.
Alternatively we could add a const version on the RandomAccessFile level which would fail, if a const size getting (e.g. without a seek) isn't possible, but it seems to me to be a potential source of hard-to-debug bugs and spurious failures. At would at least require a careful analysis of the platform support of different size getting options.