Windows Registry Size can be retrieved using WMI objects. This code snippet shows you how to get current size and maximum size for Windows registry.

ManagementObjectSearcher mgmtObjects = 

  new ManagementObjectSearcher("Select * from Win32_Registry");

foreach (var item in mgmtObjects.Get())
{
  Console.WriteLine(string.Format("Current Size: {0}MB", item["CurrentSize"]));
  Console.WriteLine(string.Format("Maximum Size: {0}MB", item["MaximumSize"]));
}

The output.