Automatic Scaling Features with D3

Use d3.geo.bounds and d3.geo.distance to compute an approximate scale to display a complete feature.

References

<html>
<head>
    <title>Automatic Scaling Features with D3</title>

</head>
<body>
    <div id="map"></div>

var width  = 600,
            height = 600;

// Create the geographic projection
        var projection = d3.geo.mercator()
            .translate([width / 2, height / 2]);

// Configure the path generator
        var pathGenerator = d3.geo.path()
            .projection(projection);

var div = d3.select('#map'),
            svg = div.append('svg'),
            grp = svg.append('g');

svg
            .attr('width', width)
            .attr('height', height);

d3.json('countries.topojson', function(error, data) {

if (error) {
                console.error(error);
                throw error;
            }

var features = topojson.feature(data, data.objects.countries).features;

// Construct a feature for South America
            // -------------------------------------

// Filter out the countries outside South America
            var southAmerica = features.filter(function(country) {
                return country.properties.continent === 'South America';
            });

var southAmericaFeature = {
                type: 'FeatureCollection',
                features: southAmerica
            };

// Compute the feature bounds and centroid
            var bounds = d3.geo.bounds(southAmericaFeature),
                center = d3.geo.centroid(southAmericaFeature);

// Compute the angular distance between bound corners
            var distance = d3.geo.distance(bounds[0], bounds[1]),
                scale = height / distance / Math.sqrt(2);

// Update the projection scale and centroid
            projection.scale(scale).center(center);

// Draws the countries
            var countries = grp.selectAll('path.country').data([southAmericaFeature]);
            countries.enter().append('path').classed('country', true);
            countries.attr('d', pathGenerator);
            countries.exit().remove();

});
    </script>
</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

Modified http://d3js.org/topojson.v1.min.js to a secure url

https://d3js.org/d3.v3.min.js

https://d3js.org/topojson.v1.min.js