For two passengers in a double cabin, everything you need for your stay fits into three cabin-sized bags (50cm x 34cm). We recommend using a soft bag for each person's clothes and a hard suitcase for more fragile items (vanity cases, electronics, etc.).
Once on the boat it will be easy to store clothes in the closets of your cabin, soft bags in the suitcase and the suitcase in the back of a closet.
If you bring too much stuff or only have hard luggage, you will lack storage space.