You also need to take into account how much shopping you plan on doing. If you anticipate getting souvenirs or heading into shops, then you need to buy a suitcase that will not only give you wiggle room in packing what you need for the trip, but will also accommodate whatever you end up bringing back. Keep in mind, that you are allowed to bring a carry-on as well as a personal item (such as a backpack.)
I could easily do a 10-day trip with a carry-on bag and personal item, but I am an expert folder and relatively low-maintenance.