On fera le calcul en utilisant deux fonctions:

• GetAttachmentsTotalSize: renvoie la taille totale occupée par les fichiers attachés à l'item;
• GetItemSize: renvoie la taille total des fichiers attachés à laquelle on y ajoute la taille du contenu inclus dans les différents champs.

GetAttachmentsTotalSize

    Public Function GetAttachmentsTotalSize(ByVal item as SPListItem) As Integer
        Dim size As Integer = 0

        For Each filename As String In item.Attachments
            Dim file As String = item.Web.GetFileAsString( _ 
                item.Attachments.UrlPrefix + filename)
            size = size + file.Length
        Next

        Return size
    End Function

GetItemSize

    Public Function GetItemSize(ByVal item as SPListItem) As Integer
        Dim size As Integer = GetAttachmentsTotalSize()

        For Each field As SPField In item.Fields
            If Not (item(field.Id) Is Nothing) Then
                size = size + item(field.Id).ToString().Length
            End If

        Next

        Return size
    End Function

Bon calcul!