I recommend a 25 to 26-inch bag. I make long trips in all 4 seasons and my 25-inch bags (I have several collected over the years) are more than adequate even in the dead of winter. A smaller bag is easier to handle, almost certainly won't go over the new draconian weight limits, costs less, and is easier to fit into small spaces in many hotel rooms.

I never take more than one coat (well, maybe an additional shell-type rain jacket, but that packs fairly small) and if snow is possible I wear snow boots on the plane. Always wear your biggest pair of shoes and don't pack more than 2 more pair. Shoes weigh a lot and take up so much space in a bag. If you develop the right packing list, you won't need a case larger than 25-26 inches.