If not i would suggest to add a function like "FsFreeSpace" and treat it like some others as not required (Do NOT export this function if it isn't supported by your plugin). RemoteInfoStruct could be reused for the size info.
could look like this:
BOOL __stdcall FsFreeSpace(char* RemoteName,RemoteInfoStruct* ri);
sorry if there is already a way to do this...

RufUsul