I thought it was around 14mm.
I have used the below pictured Rod Couplings and a Socket with Braker Bar on similar situations.
If you read the DIY with Mercedes it is handy to have a Propane Torch to heat up the area around the Plug if you have never had it out before
You also might find the size below:
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum
PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles