One example of displaying circle on google maps

index.js:

/*global google, ko*/
/*jshint esversion: 6 */
/* jshint -W031 */
(function (ns){
    "use strict";
 
    var mapOptions,
        mapCanvas,
        map,
		gpsViewModel;
 
    mapOptions = {
        zoom: 6,
        center: { lat: -34.397, lng: 150.644},
        mapTypeId: google.maps.MapTypeId.ROADMAP
    };
 
    mapCanvas = document.getElementById('map-canvas');
 
    if (!map) {
        map = new google.maps.Map(mapCanvas, mapOptions);
		gpsViewModel = new ns.GpsViewModel();
		ko.applyBindings(gpsViewModel);
		google.maps.event.addListener(map, 'click', function (evemt) {
			new google.maps.Circle({ 
				strokeColor: "#FF0000",
				strokeOpacity: 0.8,
				strokeWeight: 2,
				fillColor: "#FF0000",
				fillOpacity: 0.35,
				map,
				center: { lat: gpsViewModel.lat(), lng: gpsViewModel.lng()},
				radius: Math.sqrt(603502) * 100
			});
		});
		
		google.maps.event.addListener(map, 'mousemove', function (event) {
			gpsViewModel.lat(event.latLng.lat());
			gpsViewModel.lng(event.latLng.lng());
		});
    }

}(window.milosev));

gps.js:

/*global window, ko*/

(function (ns) {
	"use strict";
  
  var self;
	
	ns.GpsViewModel = function () {
		self = this;
		
		self.lat = ko.observable();
		self.lng = ko.observable();
		
		return {
			lat: self.lat,
			lng: self.lng
		};
	};
	
}(window.milosev));

Notice line:
radius: Math.sqrt(603502) * 100
That was just copy / paste, otherwise radius is in meters

Source download from here.

Live example:

Langitude:

Longitude: