UPDATE: I have posted and accepted a fully working solution in the answers section. Any code in this section is to be used as reference for comparison to your own NO
The best choice is to have a combined use of aspect ratio on normal definition of d3 graph's width and height. This has helped me in lot of my graph works.
Step 1 : Dynamically get the height of the div to which the graph has to be appended. 
Step 2 : Declare width as aspect ratio with respect to the dynamically caught height.
var graph_div = document.getElementById(graph.divId);
graph.height = graph_div.clientHeight;
graph.width = (960/1200)*graph.height;
The selection object is an multidimensional array, although in most cases it will probably have only one object in it. That object has a "clientWidth" field that tells you how wide its parent is.
So you can do this:
var selection = d3.select("#chart"); 
width = selection[0][0].clientWidth;
This should work:
<svg 
    xmlns="http://www.w3.org/2000/svg"
    width="860"
    height="500"
    viewBox="0 0 860 500"
    preserveAspectRatio="xMinYMin meet">
COMPLETE SOLUTION:
Here's the solution which will resize the map AFTER the user has released the edge of the window to resize it, and center it in the parent container.
<div id="mapContainer"></div>
function draw(ht) {
    $("#mapContainer").html("<svg id='map' xmlns='http://www.w3.org/2000/svg' width='100%' height='" + ht + "'></svg>");
    map = d3.select("svg");
    var width = $("svg").parent().width();
    var height = ht;
    // I discovered that the unscaled equirectangular map is 640x360. Thus, we
    // should scale our map accordingly. Depending on the width ratio of the new
    // container, the scale will be this ratio * 100. You could also use the height 
    // instead. The aspect ratio of an equirectangular map is 2:1, so that's why
    // our height is half of our width.
    projection = d3.geo.equirectangular().scale((width/640)*100).translate([width/2, height/2]);
    var path = d3.geo.path().projection(projection);
    d3.json('plugins/maps/world.json', function(collection) {
        map.selectAll('path').data(collection.features).enter()
            .append('path')
            .attr('d', path)
            .attr("width", width)
            .attr("height", width/2);
    });
}
draw($("#mapContainer").width()/2);
$(window).resize(function() {
    if(this.resizeTO) clearTimeout(this.resizeTO);
    this.resizeTO = setTimeout(function() {
        $(this).trigger('resizeEnd');
    }, 500);
});
$(window).bind('resizeEnd', function() {
    var height = $("#mapContainer").width()/2;
    $("#mapContainer svg").css("height", height);
    draw(height);
});
In d3 v4, we could do this
const projection = d3.geo.equirectangular().fitSize([width, height], geojson);
const path = d3.geo.path().projection(projection);
fitSize is equivalent to
fitExtent([[0, 0], [width, height]], geojson)
fill free to add padding