// Use a RoomFilter to find all room elements in the document. 
RoomFilter filter = new RoomFilter();

// Apply the filter to the elements in the active document
FilteredElementCollector collector = new FilteredElementCollector(document);
collector.WherePasses(filter);

// Get results as ElementId iterator
FilteredElementIdIterator roomIdItr = collector.GetElementIdIterator();
roomIdItr.Reset();
while (roomIdItr.MoveNext())
{
    ElementId roomId = roomIdItr.Current;
    // Warn rooms smaller than 50 SF
    Room room = document.GetElement(roomId) as Room;
    if (room.Area < 50.0)
    {
        String prompt = "Room is too small: id = " + roomId.ToString();
        TaskDialog.Show("Revit", prompt);
        break;
    }
}

' Use a RoomFilter to find all room elements in the document. 
Dim filter As New RoomFilter()

' Apply the filter to the elements in the active document
Dim collector As New FilteredElementCollector(document)
collector.WherePasses(filter)

' Get results as ElementId iterator
Dim roomIdItr As FilteredElementIdIterator = collector.GetElementIdIterator()
roomIdItr.Reset()
While roomIdItr.MoveNext()
   Dim roomId As ElementId = roomIdItr.Current
   ' Warn rooms smaller than 50 SF
   Dim room As Room = TryCast(document.GetElement(roomId), Room)
   If room.Area < 50.0 Then
      Dim prompt As [String] = "Room is too small: id = " & roomId.ToString()
      TaskDialog.Show("Revit", prompt)
      Exit While
   End If
End While