Do you have an existing design in mind that you absolutely love--but it is just a bit short? Or is this a hypothetical question?

There are plenty of good, and a few great. designs out there for tandem touring kayaks which will carry the weight of two paddlers and camping gear.

Shop around for a good design for a hull. Look for one which will carry the weight of all the occupants and necessary gear. Ignore what the deck looks like. You'll make the deck and then cut out most of it to give the open cockpit you want. Build a cockpit coaming around that large opening to strengthen it, and to keep out the water which may otherwise splash in.

Once you have found a good hull design you can build it as specified. That will make building it go a little faster, and you'll have the boat which matches the specs.

It matters little what your shoe size, or leg length, is when you have an open deck. You'll have plenty of foot room.Put in a couple of bulkheads to serve as floatation compartments at each end. Put hatches on them and you can store a lunch in them.

Plan to make a floating ramp so the dog can climb back aboard should it end up in the water. A piece of plywood with some scrap carpeting for traction, and a block of foam to keep one end afloat will work.

Hope this helps

PGJ