Firstly, you need to get the total no. of files. Then get size of each file.
To get total no. of files, you can use FindFirstFile/FindNextFile (FindNextFile needs to be called in a loop, until all files have been processed).
See:
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365200(v=vs.85).aspx[
^] for a sample using FindFirstFile/FindNextFile.
Then use GetFileSizeEx to get size of each file.
If you are using MFC, you can use CFileFind class instead of FindFirstFile and FindNextFile.