My 2007 1.6TDCi has a copper washer, 10mm internal diameter, 20mm outer diameter. I have been meaning to replace it for ages but never got around to it and have reused the same one for the last 8 or so oil changes.
The bigger problem I have is that I think I have striipped the thread on the plug - it tightens then suddenly gives and I need to go around another turn to tighten it again. I check the oil level regularly and it is ok, even though there is a slight seep from the drain plug. I might go to Ford to get a new one for the sake of £8 and peace of mind, and get a new washer at the same time.