I've come across several free plans on the Internet. As I see it, you need to decide what type of lighthouse you want to build. With round ones, you might be better off with sheet metal, otherwise plywood might be the better choice.
Here is a link for a free plan: http://pages.areaguides.com/ubuild/...thouse.htm
Other plans can be found at: http://www.woodworkersworkshop.com/resources/index.php?cat=638
This url may help with dimensions and style: http://www.ncsbarns.com/static/brochures/nautical%20lighthouses.pdf
The one thing I did not see in your initial posting was what style of lighthouse you are wanting to make. You really do not need a set of plans. What you may want to do is identify a lighthouse such as the Cape Hetteras Lighthouse, find its dimensions, scale the dimensions down to the height you want and then use them to build the lighthouse. After all, the full size dimensions can be regarded as inches instead of feet to help the conversion. Once you decide on a lighthouse then go to google to reasearch it and google images to get photos on it.
Here are some urls that will help you with this.