I'd go into google maps, or any other satellite map images, set it at the scale you think you want and use it as a guideline. Alternatively you can take an object in you map with a known size, like a house, draw it figure out your scale based on that known object and then keep going. (works also with bigger things, like let's say you know you want this sea to be 100km across, you can set your scale according to that and then fill out the rest.)