245/40 R17's are in short supply, you have maybe 3 companies that make that size in winter tyres.
Yes, most MB handbooks say same size all round for winter use, it's a bit of a "one for all" answer, and not always much help.
225's on 8.5" rims, which is what he has on the rear i would suspect is a bit of a stretch, it's the min size you would fit comfortably on that size rim, it is also the same setup as my front's now i have the brabus rims on, and i can confirm it is a little stretch and noticable,
As for 255/40 R17 winters, good luck with that !!
I will be advertising very shortly, my Winter Sport 3D's as i now have Bridgestone Blizzak Winter boots for my car..... but i would rather sell these as a whole set
225/45 R17 (x2) - aprox 6mm tread
245/45 R17 (x2) - aprox 5mm tread
All in good condition no damage or repairs