People who make kits do so in order to sell them. They therefore consider how many people are likely to buy them. The best selling sizes are those that fit in the back of a car. "Giant" boats are therefore for scratch builders, so, like dallas says, find some plans at whatever scale the plans are, and then either build from them scaling up as you go or get them enlarged to the size you want.
Bear in mind that Archimedes was a really clever guy when he worked out that anything floating in water displaces its own weight of water. Anything BIG is going to be HEAVY, so how the thing is going to be taken to the water, put in and subsequently extracted without either you joining it in the water, ruining your back or losing your friends should be a top consideration. Twice as long means twice as wide and twice as deep, so eight times the weight. Probably the same for the cost.