The position you get is anchored South-West, so if the entity is 2x2, your position will be the SW corner.
Use the entity size along with the anchor position of the entity to construct the area or polygon.
Entity has the getSizeX and getSizeY methods used to get the tile dimensions of an entity, tho unfortunately it's only properly implemented in the RS2Object related entities. If you want to do it for NPCs you will either need to hardcode the size, or do some hacky calculations with the model