Hi Joe, I’m sorry to say I don’t have much useful experience when it comes to taking on smaller passengers, so I can’t really say how much room they’ll need. My experience has been that you can cram a lot of people or gear into a small area fairly easily, but the amount of time it remains comfortable depends on how much room everyone has to shift around. If you’re planning on traveling long distances as a family, you’ll want more room, but if you just want to pile in and go for a little paddle now and then, you’ll be able to get away with a shorter boat. You mentioned just one person will be paddling, so maybe the most comfortable and efficient use of space would be for you and your partner to sit facing each other at opposite ends of the packraft and the two kids in between (probably with one of them nestled between the non-paddling adult’s legs and older child sitting cross-legged between the two adults’ feet). Kids have a tendency to grow though, so I’m not sure how long this arrangement would last.
Another way to get more space out of the boat might be to make thicker inflatable seats that put you up higher with your knees bent – like sitting on a canoe seat instead of a kayak seat.
I haven’t really answered your question, but hopefully I’ve given you some more ideas to help you decide what length you want to make it…
I’m not sure if there’s a limit to what length can reasonably be managed by one paddler, but even a long boat should handle alright on flat water without too much wind.