Thanks for your post. I hope you are looking forward to your Europe Winter trip!
Our winter trips stay in hostel and hotel accommodation only so a hard case suitcase is suitable. Our luggage dimensions are as follows: Each person is allowed 1 item of standard size luggage plus a day pack. The maximum weight of your main bag must not exceed 44lbs/20kgs. Size for all bags must not exceed 70cm x 45cm x 25cm.
In regards as to what to wear, a knee length dress will be fine, I would however recommend wearing some stockings/tights underneath the dress as it will be very cold outside and you may have to walk between the accommodation and restaurants.
I hope this information is helpful for you.
I hope you have a lovely time in Europe!