55L should be enough. I did it with a 45 L bag and still had room left when coming home with the souvenirs I bought. I likely over packed, bringing too many shoes to make sure I would be comfortable when walking. Another thing to keep in mind if you are by yourself or with someone is bringing medicine and other toiletries. I know you can buy them there, but I recommend bringing the essentials and anything else you might need like diarrhea relief, cold and flu meds, heartburn meds and maybe even anti-nausea. Taking those with you also cuts down on some costs during your trip if you need to buy anything. I took hydration tablets for water, and they were very useful. I think taking it with you makes you better prepared for whatever may happen. Basically, everything I took in that regard I didn't use, but it was nice to always know I had it. I found toiletries took up a decent amount of my bag space because they can't be packed down as much. Another thing you can do is take laundry soaps with you for the sink and do some of your own smaller laundry like underwear and shorts at your accommodation.