Haven't been over it myself, but CC Continental Touring Guide vol 2 says "Usually closed Nov-May. Max width 5 metres, min 2.25. Much improved and widened road with modern surface. Many hairpin bends but long level stretch on summit."
The maximum gradient on the Oberalp Pass is 10% and it is not included in the list of passes where vehicles are limited to 2.3m max. width so it can't be that bad.