I always say them as city sized. Several miles in diameter mostly and sometimes maybr twice as big in the case of somewhere like Arabasta with a lot of land surrounding the main populated areas. I don't think it takes many days to completely sail past an island if you look at the maps of the Grand Line in the anime. Took about 2 months to sail from England to the US a few hundred years ago, which is about 3000 nautical miles. 3000/60=50 so 50 miles a day in the Grand Line could be about right, Coup de Vents (or whatever they're called) aside. Dunno what data there is for sailing times in OP.