There's nothing about the geographical features of the map that could be used to objectively determine a “right” scale for either map.
So instead, decide practically: pick two points on the map and decide how long it would take to travel via a given type of transportation. Work backwards from that to determine the distance. Measure that, and translate it to a map scale.
For example, if you decided that travel on foot between Ramor and the crossroads just north of that little person icon in the second map takes half a day, and in your world travel on foot is at approximately 24 miles per day (include rest stops, etc.; see DMG p. 242), then the distance of that section of road is about 12 miles. At ~47 pixels, that gives us a scale of about 0.25 miles per pixel. That makes the continent with Ramor on it about 195 miles north-south, 270 miles east-west, or 330 miles on the long diagonal. To add a scale to your second image, then, draw a line about 40 pixels wide and label it “10 miles”. Then scale up for the world map: since the zoomed-in continent is about 1305 pixels diagonal and it's about 309 diagonal in the world map, your maps have a scale ratio of about 4.22. So on your larger map, the scale marker could be 95 pixels wide and labelled “100 miles”, assuming that you decided that original measurement takes a half-day on foot.