I think any length, it is more the style and colour that makes or breaks it, for me.
I used to think I had to wear a coat that was longer than the skirt, but life really doesn't work out like that, I see tons of people out and about with different length coats and it often really, really works.
I have a very long wool coat, minimal, dark colour, which looks great on cold, dry winter days. I only seem to struggle with raincoats, but have loosened up recently and just throw a knee length Barbour Mac on.
Short jackets look great too - a slim-is wool blazer, or velvet for occasions. For causal I have a slim fit Barbour quilted jacket in olive. It seems to go just fine with my longer skirts and dresses, especially with a nice cashmere scarf.
Denim works well in summer, but perhaps think more padded or well lined for winter.
Hoods are another matter, one thing that really doesn't work well with longer skirts/dresses in my opinion are coats like those anorak style, like you see at Seasalt, or Didriksons. Shorter raincoats such as north face and mountain equipment look a but wrong with skirts, also.
I would avoid 'technical' gear and do for something that looks thrown on - always looks better I think!
not a personal fan of biker jackets. They feel more 'city bar drinks' than everyday. But then I do live semi rural.