I don't believe that it matters a great deal what length boards you use.

I have usually used strips that are 2.4 metres (8 feet) to 3 metres (9 feet) long. I once built a 5 metre kayak using 1.8 metre strips because the only courier I could organise wanted to charge an obscene amount of money to carry anything longer than 1.8 metres. I didn't have any problems with the build and no-one has ever commented on the number of joins.

Joining strips doesn't add any extra time because I taper the ends using Nick's belt sander method and I glue them together on the forms. The joins are quick and easy and often almost invisible if I match the grain.

I find trying to work with full length strips creates a number of problems that I would prefer to avoid; problems like the glue skinning over before I can get the whole strip held in place.

Only a few of the strips will be the full length of the kayak, so it seems to me to be a lot of trouble to spend time chasing long boards when a lot of them could be shorter anyway.