Hi Jamie,
I recommend setting a distance you'd want that bay to be and then using that as a set distance to scale out the rest of your world. Googlemaps may be able to help you get a sense of how far things like cities are apart and such. Depending on what your map will be for consider how far it would take to walk, ride a horse, sail a ship or a car or plane between different locations. Often, for example, the smallest area of authority (say a county maybe) might be limited by how far one could reasonable ride a horse in a day or march soldiers.

For my dungeons and dragons maps usually walking distance is the most important aspect and I can extrapolate from that. If I want an area to be civilized the walking distance will be smaller between areas of civilization and the reverse is true. I don't tend to like players being able to traverse between different areas quickly enough that they could easily rest between encounters but that gets into a whole whack of story telling and dungeon mastering theories. Trying to replicate historical patterns is usually the safest bet!

One trick you may want to try is make your map with a grid overlaid (if you are doing it digitally). Each square in your mind you could say is a square kilometre or mile (or whatever multiple you want). That will give you a consistent and rough idea of distances.