Resize Google Maps marker icon image

Delete origin and anchor will be more regular picture

  var icon = {
        url: "image path", // url
        scaledSize: new google.maps.Size(50, 50), // size
    };

 marker = new google.maps.Marker({
  position: new google.maps.LatLng(lat, long),
  map: map,
  icon: icon
 });

As mentionned in comments, this is the updated solution in favor of Icon object with documentation.

Use Icon object

var icon = {
    url: "../res/sit_marron.png", // url
    scaledSize: new google.maps.Size(50, 50), // scaled size
    origin: new google.maps.Point(0,0), // origin
    anchor: new google.maps.Point(0, 0) // anchor
};

 posicion = new google.maps.LatLng(latitud,longitud)
 marker = new google.maps.Marker({
  position: posicion,
  map: map,
  icon: icon
 });

If the original size is 100 x 100 and you want to scale it to 50 x 50, use scaledSize instead of Size.

const icon = {
    url: "../res/sit_marron.png", // url
    scaledSize: new google.maps.Size(50, 50), // scaled size
    origin: new google.maps.Point(0,0), // origin
    anchor: new google.maps.Point(0, 0) // anchor
};

const marker = new google.maps.Marker({
    position: new google.maps.LatLng(lat, lng),
    map: map,
    icon: icon
});

MarkerImage has been deprecated for Icon

Until version 3.10 of the Google Maps JavaScript API, complex icons were defined as MarkerImage objects. The Icon object literal was added in 3.10, and replaces MarkerImage from version 3.11 onwards. Icon object literals support the same parameters as MarkerImage, allowing you to easily convert a MarkerImage to an Icon by removing the constructor, wrapping the previous parameters in {}'s, and adding the names of each parameter.

Phillippe's code would now be:

 var icon = {
     url: "../res/sit_marron.png", // url
     scaledSize: new google.maps.Size(width, height), // size
     origin: new google.maps.Point(0,0), // origin
     anchor: new google.maps.Point(anchor_left, anchor_top) // anchor 
 };

 position = new google.maps.LatLng(latitud,longitud)
 marker = new google.maps.Marker({
  position: position,
  map: map,
  icon: icon
 });