Since you mainly spend many hours sitting in the kayak, it is important that you are comfortable in it, and that it is suitably designed to fit your body.
If you’re short, choose a model that’s not too tall and wide for comfortable paddling.
If you are tall, it is more important to make sure that you have enough room to fit properly, and that the footrests/pedals are well positioned (although they are often adjustable).
Another important point is that the seat is ergonomically designed. Before buying a kayak, we therefore recommend that you try it out thoroughly.
You might find more useful tips on this sight.