■getLowestZoomLevelの利用方法
calculate best zoom to show all points on map? - Google Maps API | Google グループ
var center = new GPoint( (maxLon minLon)/2, (maxLat minLat)/2 );
var delta = new GSize( maxLon-minLon, maxLat-minLat);
var minZoom = map.spec.getLowestZoomLevel(center, delta, map.viewSize);
map.centerAndZoom(center, minZoom);