Updated December 29, 2017.
In WordPress, you can embed a YouTube video very simply by finding the “Share” link for the video and pasting it into the editing window. However, this will insert the video at the largest dimensions your website will handle. Resizing a YouTube video gives you more control over the size your video displays.
To add a YouTube video to your page at the size you want, you need to start by viewing the video on the YouTube site directly.
Under the video, click on “Share”, and then on “Embed.”
In this iframe code (circled below in green), you can see the original size (560 pixels x 315 pixels).
Copy this code into a text editor like Notepad.
Resizing a YouTube video is as simple as replacing the width and height with the size you want. Just be sure to keep the ratio correct.
Here are some other common sizes you might want to use:
300 x 169
400 x 225
640 x 360
800 x 450
Now, in a new window, open your WordPress site. Go to the page where you want to embed the video.
If the embed code uses an iframe, it’s best to insert the code into a custom field if possible.
If custom fields are not available, you can paste it directly into the editing window after clicking on the “Text” tab.
Be sure to click to update your page after making your changes.