Или с использованием родных возможностей
jsfiddle.net/a36pq2L0var obj = {
"0":["2","2","2","2"],
"1":["2","0"],
"28":["0","0","0","0"],
"29":["0","2"],
"30":{"3":"2","0":"2","14":"2","6":"1"},
"-2":{"8":"2","2":"1","3":"1"},
"36":{"27":"2","15":"1","16":"1"},
"-83":{"67":"2"},
"194":{"214":"2"},
"178":{"192":"2"},
"127":{"145":"2"},
"55":{"147":"2"},
"47":{"21":"1"},
"40":{"19":"1"},
"37":{"19":"1"},
"65":{"17":"1"}};
var arr=Object.keys(obj), stack=[]
arr.forEach(function(key){
var en=obj[key];
stack.push.apply(stack,Object.prototype.toString.call(en)=='[object Array]'?en:Object.keys(en));
})
var xMin=Math.min.apply(null, arr),
xMax=Math.max.apply(null, arr);
yMin=Math.min.apply(null, stack);
yMax=Math.max.apply(null, stack);
alert("["+xMin+", "+xMax+"]\n ["+yMin+","+yMax+"]");