Fronts on an R230 should be 255/35 R19.Our 230 (and 129 for that matter) is on 19" split rim AMG rims. The normal size for 19swoukd be 285/30 rear and 245/35 on the front. The previous owner fitted 285/35 and 245/40 to soften the ride slightly and to be fair it isn't bad at all.
The 129 would properly have 235/35s but I fitted 255/35 and that similarly improved matters to make it tolerable.
You're correct (I was going from memory). On ours they're 255/40.Fronts on an R230 should be 255/35 R19.