try this
using System;
using System.IO;
class Program
{
static void Main()
{
const string fileName = "test.txt";
FileInfo f = new FileInfo(fileName);
long s1 = f.Length;
File.AppendAllText(fileName, " More characters.");
FileInfo f2 = new FileInfo(fileName);
long s2 = f2.Length;
Console.WriteLine("Before and after: " + s1.ToString() +
" " + s2.ToString());
long change = s2 - s1;
Console.WriteLine("Size increase: " + change.ToString());
}
}