Is it sump or carter by the way?
That is just the difference in languages.
In English we say 'the oil sump' or simply 'the sump', but sometimes the term used could also be 'the sump pan'.
In French 'oil pan' is 'carter d'huile' so you can see why 'carter' is a term for the sump and a sump plug (or oil pan plug) in French would be bouchon de carter d'huile.
Either way. I'm looking for the size so I can buy spare ring seals instead of having to buy a whole assortment and ending up with a bunch of unused ones that just sit and collect dust for eternity.
Planning to do the oil change this weekend and last time I didn't order the correct size I think so I ended up re-using the old one which worked well enough but I don't want to risk that again.
I answered this question about sump plugs in one of your earlier postings, in May 2021! This is why each posting should stick to one topic and not mix things as I have said before.
If you have a flat copper washer on the sump plug, then it is perfectly OK to re-use it as long as it is not damaged or degraded in any way, such as being bent, or has a scored surface. This car did not use a crushable copper washer, such as Renault often use on their sump plugs. You cannot re-use a crushable copper washer. After they have been crushed once when you tighten the plug, they deform to the shape of the joint. They cannot be used a second time as they would no longer be able to be deformed to match the joint a second time.
As for diameter, whoever suggested an inner diameter of 10mm and an external diameter of 13.5mm is totally wrong! In the first place the difference is just 3.5mm which means the width of the washer would be just 1.75 mm. Rubbish and that diameter is far too small.
The width of the copper washer should be about 3mm which means the external diameter is 6mm more than the internal one.
I don't have one to hand as mine are in my garage, but I'm sure the internal diameter is roughly 20mm and the external diameter 26mm, and they should be 1mm thick. You could also use a suitable Dowty sealing washer, which is a washer that has a rubber seal all around the internal diameter. I keep some of both in my stock, but obviously the Dowty washers are slightly more expensive than simple copper ones.
Roy