If you’ve been working with WordPress for awhile, you know how easy it is to embed a YouTube video in a page or post. Simply paste the video URL on it’s own line, publish the post, and you’re done. Occasionally, however, the size of the resulting video may not fit the main content area of the theme you’re using–the result can be a video that’s only partially visible in the player.
If you’re having this problem, here’s how to fix it: Visit the Dashboard and click Settings > Media and scroll to the Embeds section of the page as shown here:
Change the width attribute to a dimension that fits the width of the main content area on your site, which might require a little experimentation to determine, and leave the height attribute blank (this will cause the height of the video player to scale appropriately).
Likewise, you can do the same for images and thumbnails at the top of the page by changing the default sizes for those. This can be very helpful when you’re using WordPress galleries and need the thumbnails to be something other than 150px by 150 px. If you would like to change the thumbnail sizes in your galleries, however, make sure to do that in the Settings > Media before you create the gallery.