Embedding

You can embed a wide range of ‘embeddable’ content such as videos and images in to your WordPress website simply by copying and pasting the URL in to your web page.

So to embed a YouTube.com video in your webpage, just copy the URL of the video and paste it directly in to your web page on its own line.

Here is a list of the content you can embed in WordPress this way.

WordPress will automatically turn those URLs into an embed when the post is viewed.

Embedded Media Size/Dimensions

The default size of the embedded media is defined by the setting in Dashboard → Settings → Media → Embeds.

Alternatively, if you wish to customise the size of one embedded video while keeping the others at the default size, you can do so by wrapping the URL in an [embed] shortcode and its width and height parameters:

[embed width="123" height="456"]URL[/embed]

For example:

[embed width="123" height="456"]http://www.youtube.com/watch/?v=12324343434343[/embed]

More information: Embeds – WordPress Codex