在应用程序设置中设置固定高度参数后，

您可以调用 FB.Canvas.setSize 来更改页面高度。

After you setup the fixed height parameter in your application settings,
you can call FB.Canvas.setSize to change the page height.

如果您的页面高度动态变化，则可以运行 FB.Canvas.setDoneLoading

来检查Canvas是否已加载并运行 FB .Canvas.setAutoGrow 。

If your page height changes dynamically, you can run FB.Canvas.setDoneLoading
to check if Canvas is loaded and run FB.Canvas.setAutoGrow.

window.fbAsyncInit = function() {
    // assume we are already logged in
    FB.init({ appId: '<?=$fb_app_id ?>', xfbml: true, oauth: true, cookie: true });

    FB.Canvas.setSize({ width: 810, height: 950 });

    FB.Canvas.setDoneLoading( function(response) {
        console.log(response.time_delta_ms);
        FB.Canvas.setAutoGrow();
    });
}

FB文档： https://developers.facebook.com/docs/reference/javascript/FB.Canvas.setDoneLoading/