I think I will build the hull and mock something up for the deck using cardboard or somethink like that. That should give a pretty good impression of what the seating position will be like.

Excellent.

I am also still deciding om whether to go small ocean cockpit and keyhole. My experience so far has been that I can hardly get out of kayaks with too small a cockpit without falling over and getting wet. I don't think wet exits will be a broblem. It is the dry ones that will, especially when I am tired from a long day of paddling. How do you like your cockpit?

It's very nice to be able to enter you boat easily on the water or to clamber out quickly when you hit the beach.

You'll find a lot of enthusiasm here (Et, I know you are watching! :-) ) for small cockpits. Bjorn is also a fan of small cockpits in most kayaks.

I won't paddle a boat with a small cockpit, after building a couple (Outer Island, and a short Greenland 'rolling boat') and paddling them. On the few occasions when I've paddled with somebody using a small cockpit boat, the individual has been an expert roller, so I had no concerns about rescues.
In my area (Vancouver Island), 95% of the kayaks I see on the water have 'standard' cockpits.

It's very nice to 'have the deck in your lap' so your chart and other 'stuff' is close by, but for me that's the only positive thing about a small cockpit. If you design/build the coaming lip poorly, it's probably better to use a small cockpit, as the spraydeck/sprayskirt 'holds' better on the smaller, rounder coaming.

I like the NDK 'keyhole' style cockpit and I use that shape on boats I build. The Mariner cockpit shape also works well. With both those cockpit shapes, the deck is 'rolled down' to provide a good base for thigh support.
I'm not a big fan of more 'oval' openings with thigh hooks added, though some of the commercial designs with movable thigh hooks work well.
Remember that it's not just a matter of adjusting the footpegs if you decide to put a much shorter or taller person into your boat - the thigh contact points are important. Shorter paddlers often end up with their thighs out of contact with the boat and their knees under the thigh padding.

Getting the cockpit in the right position relative to the seat (and balance point of the boat) is important, so take that into account. Bjorn's designs have the seat incorporated into the cockpit coaming aft. If you are going to use a different seat arrangement (say, with a couple of inches of space between the seat back or backrest and the coaming), you need to 'set' the cockpit opening in the appropriate position.

If you want to use a high-angle paddle stroke and body rotation, having the freedom to put your knees in the center of the cockpit in calmer conditions is a big advantage.

You will get a lot of different opinions on this!