I had a similar issue, building an 18' boat in a 18' x 10' shed. I arranged the boat corner to corner which gave me a couple extra feet. It still wasn't a lot of room to maneuver around in but doable.

Might sound silly, but: If you do end up building in a small spot, corner to corner, make certain that you can get the boat out when completed.

Barry has a good point. I should have mentioned that I also had to add another door to the end of the shed to get it out. An added bonus is that you can open this door on nice days and arrange the boat so that one end sticks out the door ... makes it easier to get around it.

I had a similar issue in the my 17' boat was built in a 17' room.  A little made it work until hull #2 was layed on the 18' strong back... uncomfortable but workable.  One thing to keep in mind is the relative times, building vs. enjoying.  while building does take a substantial amount of time, keep in mind that it is a mere blink compared to the years of enjoyment, so duck under and find a way to make it work so that you can end up with the boat you want as opposed to 15.5/16ths the boat you want.