open System.IO

type FolderStatistic = { Name : string
                         Size : int64 }

let calculateFolderSize (folder : string) =
    Directory.GetFiles folder
    |> List.ofSeq
    |> List.sumBy (fun f -> FileInfo(f).Length)

let subfolders : DirectoryInfo array =
    let folder = new DirectoryInfo(@"C:\\tmp")
    folder.GetDirectories()

subfolders
|> List.ofArray
|> List.map (fun x -> { Name = x.Name; Size = calculateFolderSize x.FullName })
|> List.sortByDescending(fun x -> x.Size)