It's pretty small.
http://cdn.bulbagarden.net/upload/b/b6/Unova_world_map.png ^This map shows the visible ports of Unova.
By using blocks on the grid and assuming each grid=2m I estimated that Caselia is only 148m in wide.
Using the map I linked I used Castelia's width to estimate that the circumference of the entire Pokémon world must be 5040m which is only 8 times smaller than earth.