Thanks Matt! Really appreciate the reply, even if you don’t have a definitive length for me. I think confirming that 1 paddler could handle a long boat is helpful. If anything it seems the longer boat will fare better on the mellow lakes and rivers we’ll use (your “Tracking, or Directional Stability” article makes that point with regard to kayaks/canoes).
Also a good point about getting up higher from the floor. I’m planning on making it self bailing, and was going to aim for a total height of 10 or more inches from the bottom of the boat (using a combination of inflatable floor and inflatable seats). Bruce in his “Voyageur 2-person first float” article talked about getting 10″ up and being comfortable paddling there. I know the Alpacka Forager has a 7″ inflatable floor.
Obviously the weight will go up as it gets longer, but activities with kids don’t tend to be lightweight. Bruce’s Voyageur was 72″ and only weighed 8 1/4 lbs, so it’s still possible to keep weight reasonable. That’s the beauty of DIY.
Now I’ll just have to figure out how to secure the floor to the raft. It will be a separate “camp-mattress” type floor. And I’ll also have to work out where the drains will be in the floor and how many.