Well, first you need to have a good idea of what you want to make. This is your next project, a comfy little sloop, as shown in the following screenshot:

You will need something to help you tell what size the object you are building will be. There are many ways to get the measurements you need. You've already tried some of these methods.

Quite often, you can find plans on the net on websites such as http://www.boatdesign.net/plans/index.htm. You can also measure the object yourself. Once, to build a model of an electric guitar, I went to the Carvin guitar factory, took my caliper micrometer and rulers to measure the dimensions of one of their Ultra V guitars, and made some accurate drawings to guide my modeling.

In the plans for the sloop, as shown in the next screenshot, you will see a top view and a side view, and to the very right of the side view, the number 6 is present between two dashes to indicate the scale of the drawing. In this case, the lines above and below...