get the file block size from system.

First of all, there is no a single block size. This is a characteristic if each volume, not of the whole system.
 
This is not trivial thing at all. I prefer to show how to do it with WMI, because, this way, you can do it in pure C#.
 
Consider the following code which shows you the block size of each volume:

using System.Management;
 
//...

            ManagementObjectSearcher searcher = new ManagementObjectSearcher("SELECT * FROM Win32_Volume");
            ManagementObjectCollection collection = searcher.Get();
            foreach (ManagementObject item in collection) {
                System.Console.WriteLine("Name: {0}", item["Name"]);
                System.Console.WriteLine("Block size: {0}", item["BlockSize"]);
                System.Console.WriteLine();
            } // loop

 
How it works?
 
Look at the class ManagementObject:
http://msdn.microsoft.com/en-us/library/system.management.managementobject.aspx[^].
 
As you can see, this class provides access to different properties of a run-time management object via the indexed property '[]', indexed by strings. How do you know what property to use?
The class provides reflective API; for example, you can use the property Properties (and Methods, so you can invoke methods). This class is actually a wrapper over some "real" management objects you can get via a query.
 
But where to read about the meaning of the properties and method? You can use C++ documentation to get the properties you want. For Win32_Volume, use
http://msdn.microsoft.com/en-us/library/windows/desktop/aa394515%28v=vs.85%29.aspx[^].
 
Good luck,

—SA