There is no API to get the total size of a specific directory in the isolated storage. Therefore, the only alternative you have is to browse the files and manually compute the total size.

Here is a sample implementation:

　　long total = 0;

　　using (var isolatedStorage = IsolatedStorageFile.GetUserStoreForApplication())

　　{

　　　　 string folder = "folder/";

　　　　foreach (var fileName in isolatedStorage.GetFileNames(folder))

　　　　{

　　　　　　 using (var file = isolatedStorage.OpenFile(folder + fileName, FileMode.Open))

　　　　　　 {

　　　　　　　　  total += file.Length;

　　　　　　 }

　　　　  }

　　}

　　MessageBox.Show(total + " bytes");