Here’s how to get ("calculate") the size of a folder using .NET 4 and LINQ. The code also includes the size of all sub-directories.

using System;
using System.IO;
using System.Linq;

namespace ConsGetDirSize
{
   internal class Program
   {
      private static long DirectorySize(DirectoryInfo dInfo, bool includeSubFolders)
      {
         // enumerate all the files
         long totalSize = dInfo.EnumerateFiles()
                      .Sum(file => file.Length);

         // if SubDirectories should be included
         if (includeSubFolders)
         {
            // enumerate all sub-directories
            totalSize += dInfo.EnumerateDirectories()
                     .Sum(dir => DirectorySize(dir, true));
         }
         return totalSize;
      }

      private static void Main(string[] args)
      {
         DirectoryInfo dInfo = new DirectoryInfo(@"E:\Videos");

         // set bool parameter to false if you do not want to include subdirectories.
         long sizeOfDir = DirectorySize(dInfo, true);

         Console.WriteLine("Directory size in Bytes : " + "{0:N0} Bytes", sizeOfDir);
         Console.WriteLine("Directory size in KB : " + "{0:N2} KB", ((double)sizeOfDir) / 1024);
         Console.WriteLine("Directory size in MB : " + "{0:N2} MB", ((double)sizeOfDir) / (1024 * 1024));
         Console.WriteLine("Directory size in GB : " + "{0:N2} GB", ((double)sizeOfDir) / (1024 * 1024 * 1024));
         Console.ReadLine();
      }
   }
}

Thanks to DirectoryInfo.EnumerateDirectories and DirectoryInfo.EnumerateFiles you don't have to iterate for yourself over all the files, getting the size of the directory content within the blink of an eye.