How to change width of CubeGeometry with Three.js?

Scale properties can be used to for changing width, height and and depth of cube.

    //creating a cube 
    var geometry = new THREE.BoxGeometry(1,1,1);
    var material = new THREE.MeshBasicMaterial({color:"white"});
    var cube = new THREE.Mesh(geometry, material);


    //changing size of cube which is created.
    cube.scale.x = 30;
    cube.scale.y = 30;
    cube.scale.z = 30;

You can dispose the geometry of cube and affect the new one like this :

let new_geometry = new THREE.CubeGeometry(500,200,200);
geometry.dispose();
cube.geometry = new_geometry;

Just to complete comment and solution from question (and have an answer present with example code):

// create a cube, 1 unit for width, height, depth
var geometry = new THREE.CubeGeometry(1,1,1);

// each cube side gets another color
var cubeMaterials = [ 
    new THREE.MeshBasicMaterial({color:0x33AA55, transparent:true, opacity:0.8}),
    new THREE.MeshBasicMaterial({color:0x55CC00, transparent:true, opacity:0.8}), 
    new THREE.MeshBasicMaterial({color:0x000000, transparent:true, opacity:0.8}),
    new THREE.MeshBasicMaterial({color:0x000000, transparent:true, opacity:0.8}), 
    new THREE.MeshBasicMaterial({color:0x0000FF, transparent:true, opacity:0.8}), 
    new THREE.MeshBasicMaterial({color:0x5555AA, transparent:true, opacity:0.8}), 
]; 
// create a MeshFaceMaterial, allows cube to have different materials on each face 
var cubeMaterial = new THREE.MeshFaceMaterial(cubeMaterials); 
var cube = new THREE.Mesh(geometry, cubeMaterial);

cube.position.set(0,0,0);
scene.add( cube );
cube.scale.x = 2.5; // SCALE
cube.scale.y = 2.5; // SCALE
cube.scale.z = 2.5; // SCALE

A slightly advanced, dynamic example (still the same scaling) implemented here: