Phaser.io 3: Get game size in scene

The following should work:

scene.sys.game.scale.gameSize

You can use the default camera:

ship.setPosition(this.cameras.main.centerX, this.cameras.main.centerY);

From the Phaser 3 API Documentation on Camera:

Cameras, by default, are created the same size as your game, but their position and size can be set to anything.

In a scene, in the preload() and create() methods (not in the constructor) you can access the canvas element with this.sys.game.canvas. So to get the canvas size, you can do:

create() {
    let { width, height } = this.sys.game.canvas;
}

For my part I like to add the following code to ease the access to the canvas:

preload() {
    this.canvas = this.sys.game.canvas;
}