For reference...

The StringBuilder's initial capacity is set to 16 by default.  The capacity then increases by an order of 2 whenever the current capacity is passed.  Therefore, if the String you append to the StringBuilder causes the capacity to expand, it will increase as seen below:

   StringBuilder sb = 
      new System.Text.StringBuilder();
   int capacity = sb.Capacity;

   for (int i=0; i<5000; i++)
   {
    sb.Append("a");
    if (capacity != sb.Capacity)
    {
     PrintLine("Capacity:  " + sb.Capacity);
    }
    capacity = sb.Capacity;
   }

results in the following capacity adjustments:
Capacity: 16
Capacity: 32
Capacity: 64
Capacity: 128
Capacity: 256
Capacity: 512
Capacity: 1024
Capacity: 2048
Capacity: 4096
Capacity: 8192