the std.stdio.File struct should have a way of getting the file size. If the file is a standard stream or something, some convention like returning -1 could be used. This would be trivial to implement (though I'm not saying this is necessarily the best or most efficient way) by using something like:
try {
std.file.getSize(this.name);
} catch(FileException) { // A stream that isn't a "real" file.
return -1;
}