I'd probably pick Berlin as one end of your trip and Milan as the other (but I'd change the Italian city based on where I found the best flight deal into or out of). I'd pick my direction, since it's all in December, based on the better flight options.
This time of year I'd prefer the train. The weather is unpredictable in winter and Brisbane doesn't prepare you very well for winter driving.
For length of stays, I always look at guidebooks to get an idea what I want to see. I schedule only two major sights in a day but keep a list of lesser sights I might like to see if I have time. I normally divided my must-sees by two, the number of days I want to stay, and add one, the number of nights I need to stay to have that many days.
Remember days will be quite short this time of year.
Count the days on either side as travel days. Even if your travel leg is pretty short, a large part of your day will be consumed by packing, checking out, getting to the station, catching the train, making connections, getting to your next hotel, checking in, and unpacking/getting settled. Sometimes you will have enough time to see a sight on arrival day, but it's better not to plan for it.