Our group consisted of two families, two parents plus one 3-year-old each, so six people in total. We stayed for 4 nights.
The size of the house is perfect for this kind of group, we had plenty of space and the garden (including sand pit and swing) was fantastic for the kids. Communication was easy (we had a Polish native speaker in the group though so can't say much about conversing in other languages) and we got some really nice recommendations from the landlord, like for example which beach to go to, this was really helpful. The Polish baltic coast is so beautiful! All in all it was a really nice and positive experience!
Most destinations (the sea, proper supermarket, etc) are roughly 20-30 minutes drive by car, which is what we expected. We were surprised (and very happy) to find towels provided as the agency had told us otherwise (maybe make it explicit in the advert?) We didn't use Sauna or pool as it was really hot so can't say anything about that.
One thing we missed was a dishwasher. We knew there wasn't one but in case the landlord is wondering what to improve to make things even better, this would be top of our list. Second would be proper black out curtains. Our daughter kept waking up fairly early as it was hard to keep the rooms dark. That would also potentially help with temperature in summer as it does get quite hot upstairs where the bedrooms are.
Thanks again, we had a really nice time!