Selecting the right size is the biggest consideration when copying a YouTube widget for placement in your Tumblr sidebar. When you click the "Share" link on a YouTube video page and select "Embed," YouTube offers several size options. If you don't know the width of your Tumblr sidebar, the most practical solution is to copy the widget using the default settings, and then adjust the widget size after you insert it into your theme.

Adjust Widget Size

When you first paste the YouTube widget into the Description area of your blog's Customize page, the video automatically appears on your sidebar in the preview pane. If the video is too big or too small, change the "width" and "height" values inside the widget. You may need to play around with the settings to get it right. After each change, Tumblr automatically changes the size of the video in the preview pane. Click "Save" after you find the perfect setting.