Program to calculate size of map and point to point distance.

[Elros]

Your island is less than about 400 miles across: the errors inherent in the map image will overwhelm any likely measurement errors relating to projection. In short, because the island is smallish, you can mostly ignore the projection. If the world itself is flat, then projection is an irrelevant concept.

Now that your map is declared as something that can be measured with a ruler, you need a tool that will measure those distances. There are many tools out there that will do that (try searching for "measure distance on an image web page" in your favorite search engine. Then define that one pixel in your image is X real-world units as shown by your scale bar.

So the dimension are important?
I make this island tiny, but for my book i thinking somethin like europe dimension, if i make that maps the dimension of measure is a problem?

A less complex technique use a physical ruler to measure the scale bar and your desired path directly onscreen, then do the math to convert scale units to real-world units. This also works if you print out the map. The scale bar is the critical part here.

Yea is more simply this method, but i have to decide the scale bar of the map before, right?