A degree of longitude at the equator measures roughly 111km. A degree marker is indicated on one side of the maps, but is beyond reach of the maps on the other side, so we're looking at a maximum island width of 70km or so.
A Boeing 737-700 ER could easily reach all cities depicted on the previous map, it has a range of around 11,000km. Such an aircraft requires a runway of 1683m. If such a runway were to fit within the circle designated as the airport, almost completely crossing it's diameter, and then we can probably safely say that the airport's circle is 2km across.
These numbers give a rough scale and the upper and lower limits to work within.
If the airport's circle is 2km across, then the main city's circle is about 2.5km across. Converted to the city blocks depicted, that makes them about 150m on each side of their squares. It also scales the sports stadium at the centre of the depicted city in such a way that the centre field is roughly 120-150m long by 60-75m wide. Which is larger enough to accommodate a rugby field (and I'd expect most other sporting field would fit in that area).
If the white line marked at the centre is 3km wide, then the fainter line is 60km wide. The whole archipelago is about 35-40km wide.
I'm happy with that.