The first thing I think about is how much detail I want to put into the map, more detail = bigger image size...then I start with a 4,000 x 4,000 image because I can always crop it down. Next I just put down some roads and a coast/river if there is one. The width of the road determines the size of the building shapes. So if my road is drawn with a 9-pixel brush and I want these roads to be main streets then these 9 pixels will roughly equate to a 20 foot wide road (to allow for two carts to go by plus some foot traffic). Thus, my buildings will have to be sized accordingly. If this scale isn't working then I scrap the roads and start over with a different brush and I haven't wasted too much time yet. The terrain features will come later, I just want to get a good layout with approximate scale. Third, I rough in my castle "footprint", cathedral footprint, and some huts and houses. If they look like they fit the roads then I take a few days and put more buildings in.

As to your map, when I look at it, it looks like many miles maybe hundreds of miles. You need more cues to get the visual scale right...roads, buildings, etc. The towers you currently have would make this a small village if done all the way to the end because there isn't enough space. So, just make them smaller, put in some roads, extend that light blue area of the sea, take out the coastal lines, and then see where you stand.