What boat for a large paddler?
There are two ways of being larger, you could be tall and have trouble fitting into most kayaks, or you could be heavy and need a design with more displacement for efficient paddling. Tall paddlers may have trouble getting in and out of small cockpits. This can generally be mitigated by making the cockpit larger. If you are making your own boat, you can control the size of the cockpit opening. The other problem tall people can have is foot room. Big feet at the end of long legs can jam your feet down into the smaller part of the boat. Generally the kayak gets narrower and lower the farther forward you go. Long legs can make the problem of large feet worse as your feet end up in this smaller part of the boat.
Heavier paddlers are often concerned about fitting in the cockpit also. Not to say that this is never a problem, but most people are not really as wide as they think they are. Many men tend to be large above the waist, but still fit into most cockpits without a problem. While women may have larger hips, being generally smaller overall, they still don't have a problem fitting in the standard cockpit.
The bigger concern of larger paddlers is whether they are going to sink the kayak. Forget about it. It takes a lot to sink a kayak. Even the Little Auk would require over 600 lbs to sink it. What you should consider is how efficiently the boat will operate. A boat designed for a smaller paddler will tend to bog down if it is overloaded. You want a boat designed to carry heavier loads efficiently.
Some designs are intended for touring with gear. These designs will work for heavier paddlers, but would not have as much excess capacity for gear. Other designs will maintain their efficiency even when loaded beyond their intended displacement. Don't get too caught up with the design displacement listed for each boat. These are merely a starting point. Most boats will work well loaded beyond their design displacement.
The Great Auk is designed to carry a lot of gear and responds well to heavy loads. The Expedition Single is also designed to carry gear and will work well for heavier paddlers. The Guillemot "L" is specifically intended for larger paddlers who want to carry a fair amount of gear.
It would seem that the High Capacity Great Auk would be the ideal boat for bigger guys, and it was in fact designed for a large paddler, but it really needs a big load to operate efficiently. It is really intended for a big paddler who also wants to carry a lot of gear. Unless you truly need a very large boat you may want to consider one of the other designs.
Taller paddlers who aren't particularly heavy may want to consider the Expedition Single or the higher decked Night Heron. These boats have pretty good foot room. The Great Auk and 14' Great Auk also has good foot room due to their width.