There are several map sizes starting with the default of 30m square. Grow Map doubles the map size each time, up to 240m. Doubling the map takes couple seconds to move/copy the data from the old map into the new map. Larger maps are a bit slower to process and render on your phone, but not too bad.
It is recommended to start mapping from the center of the building to make the most efficient use of current map size. If you find yourself mapping at the edge of the map, just grow the map and continue. Pinch-zoom works as expected, but does not grow size of the map.