[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
static extern bool GetDiskFreeSpace([MarshalAs(UnmanagedType.LPTStr)]string rootPathName,
ref int sectorsPerCluster, ref int bytesPerSector, ref int numberOfFreeClusters, ref int totalNumbeOfClusters);

public static DiskInfo GetDiskDiskInfo(string rootPathName)
{
DiskInfo diskInfo = new DiskInfo();
int sectorsPerCluster = 0, bytesPerSector = 0, numberOfFreeClusters = 0, totalNumberOfClusters = 0;
GetDiskFreeSpace(rootPathName, ref sectorsPerCluster, ref bytesPerSector, ref numberOfFreeClusters, ref totalNumberOfClusters);

diskInfo.RootPathName = rootPathName;
diskInfo.SectorsPerCluster = sectorsPerCluster;
diskInfo.BytesPerSector = bytesPerSector;
diskInfo.NumberOfFreeClusters = numberOfFreeClusters;
diskInfo.TotalNumberOfClusters = totalNumberOfClusters;

return diskInfo;
}

public struct DiskInfo
{
public string RootPathName;
public int SectorsPerCluster;
public int BytesPerSector;
public int NumberOfFreeClusters;
public int TotalNumberOfClusters;
}

//Forgot to add this on my initial post, and I just noticed.
public static int GetActualFileSize(FileInfo file)
{
double clusterSize = 0;
double numberofClusters = 0;
int actualSize = 0;
DiskInfo diskInfo = new DiskInfo();
diskInfo = GetDiskInfo(file.Directory.Root.FullName);

[color=#FF0000]clusterSize = (diskInfo.BytesPerSector * diskInfo.SectorsPerCluster);[/color]

//Added this, seems if the file length and clustersize are exact it would over-report the size
if ((file.Length % clusterSize) > 0)
clusterSize += 1;

numberofClusters = Math.Round(file.Length / clusterSize);

actualSize = (int)(numberofClusters * clusterSize);
return actualSize;
}